diff --git a/R/coerce.R b/R/coerce.R
index 01e4682..b8e3563 100644
--- a/R/coerce.R
+++ b/R/coerce.R
@@ -12,7 +12,6 @@
 #' 
 #' @seealso [coerce_to_sparse_data_frame()] [coerce_to_sparse_tibble()]
 #' @examplesIf rlang::is_installed("Matrix")
-#' set.seed(1234)
 #' sparse_tbl <- lapply(1:10, function(x) sparse_double(x, x, length = 10))
 #' names(sparse_tbl) <- letters[1:10]
 #' sparse_tbl <- as.data.frame(sparse_tbl)
@@ -30,8 +29,32 @@ coerce_to_sparse_matrix <- function(x) {
     )
   }
 
+  if (!all(vapply(x, is.numeric, logical(1)))) {
+    offenders <- which(!vapply(x, is.numeric, logical(1)))
+    offenders <- names(x)[offenders]
+    cli::cli_abort(c(
+      x = "All columns of {.arg x} must be numeric.",
+      i = "Non-numeric columns: {.field {offenders}}."
+    ))
+  }
+
+  if (!any(vapply(x, is_sparse_numeric, logical(1)))) {
+    res <- as.matrix(x)
+    res <- Matrix::Matrix(res, sparse = TRUE)
+    return(res)
+  }
+
+  if (!all(vapply(x, sparse_default, numeric(1)) == 0, na.rm = TRUE)) {
+    offenders <- which(vapply(x, sparse_default, numeric(1)) != 0)
+
+    for (i in offenders) {
+      x[[i]] <- x[[i]][]
+    }
+  }
+
   all_positions <- lapply(x, sparse_positions)
   all_values <- lapply(x, sparse_values)
+
   all_rows <- rep(seq_along(x), times = lengths(all_positions))
 
   all_positions <- unlist(all_positions, use.names = FALSE)
@@ -78,6 +101,11 @@ coerce_to_sparse_tibble <- function(x) {
     )
   }
 
+  if (!methods::is(x, "dgCMatrix")) {
+    x <- as(x, "generalMatrix")
+    x <- as(x, "CsparseMatrix")
+  }
+
   if (is.null(colnames(x))) {
     cli::cli_abort(
       "{.arg x} must have column names."
@@ -119,6 +147,11 @@ coerce_to_sparse_data_frame <- function(x) {
     )
   }
 
+  if (!methods::is(x, "dgCMatrix")) {
+    x <- as(x, "generalMatrix")
+    x <- as(x, "CsparseMatrix")
+  }
+
   if (is.null(colnames(x))) {
     cli::cli_abort(
       "{.arg x} must have column names."
diff --git a/man/coerce_to_sparse_matrix.Rd b/man/coerce_to_sparse_matrix.Rd
index 9f8bc2c..673b67d 100644
--- a/man/coerce_to_sparse_matrix.Rd
+++ b/man/coerce_to_sparse_matrix.Rd
@@ -20,7 +20,6 @@ a sparse matrix out of a dense data frame is not ideal.
 }
 \examples{
 \dontshow{if (rlang::is_installed("Matrix")) (if (getRversion() >= "3.4") withAutoprint else force)(\{ # examplesIf}
-set.seed(1234)
 sparse_tbl <- lapply(1:10, function(x) sparse_double(x, x, length = 10))
 names(sparse_tbl) <- letters[1:10]
 sparse_tbl <- as.data.frame(sparse_tbl)
diff --git a/tests/testthat/_snaps/coerce.md b/tests/testthat/_snaps/coerce.md
new file mode 100644
index 0000000..4160978
--- /dev/null
+++ b/tests/testthat/_snaps/coerce.md
@@ -0,0 +1,81 @@
+# coerce_to_sparse_matrix() errors on wrong input
+
+    Code
+      coerce_to_sparse_matrix(1:10)
+    Condition
+      Error in `coerce_to_sparse_matrix()`:
+      ! `x` must be a <data.frame>, not an integer vector.
+
+---
+
+    Code
+      coerce_to_sparse_matrix(matrix(0, nrow = 10, ncol = 10))
+    Condition
+      Error in `coerce_to_sparse_matrix()`:
+      ! `x` must be a <data.frame>, not a double matrix.
+
+---
+
+    Code
+      coerce_to_sparse_matrix(iris)
+    Condition
+      Error in `coerce_to_sparse_matrix()`:
+      x All columns of `x` must be numeric.
+      i Non-numeric columns: Species.
+
+# coerce_to_sparse_matrix() materializes non-zero defaulted columns
+
+    Code
+      res <- coerce_to_sparse_matrix(sparse_df)
+    Output
+      sparsevctrs: Sparse vector materialized
+      sparsevctrs: Sparse vector materialized
+
+# coerce_to_sparse_data_frame() errors with no column names
+
+    Code
+      coerce_to_sparse_data_frame(sparse_mat)
+    Condition
+      Error in `coerce_to_sparse_data_frame()`:
+      ! `x` must have column names.
+
+# coerce_to_sparse_data_frame() errors with wrong input
+
+    Code
+      coerce_to_sparse_data_frame(mtcars)
+    Condition
+      Error in `coerce_to_sparse_data_frame()`:
+      ! `x` must be a <sparseMatrix>, not a data frame.
+
+---
+
+    Code
+      coerce_to_sparse_data_frame(1:10)
+    Condition
+      Error in `coerce_to_sparse_data_frame()`:
+      ! `x` must be a <sparseMatrix>, not an integer vector.
+
+# coerce_to_sparse_tibble() errors with no column names
+
+    Code
+      coerce_to_sparse_tibble(sparse_mat)
+    Condition
+      Error in `coerce_to_sparse_tibble()`:
+      ! `x` must have column names.
+
+# coerce_to_sparse_tibble() errors with wrong input
+
+    Code
+      coerce_to_sparse_tibble(mtcars)
+    Condition
+      Error in `coerce_to_sparse_tibble()`:
+      ! `x` must be a <sparseMatrix>, not a data frame.
+
+---
+
+    Code
+      coerce_to_sparse_tibble(1:10)
+    Condition
+      Error in `coerce_to_sparse_tibble()`:
+      ! `x` must be a <sparseMatrix>, not an integer vector.
+
diff --git a/tests/testthat/test-coerce.R b/tests/testthat/test-coerce.R
new file mode 100644
index 0000000..cbd3e71
--- /dev/null
+++ b/tests/testthat/test-coerce.R
@@ -0,0 +1,205 @@
+### coerce_to_sparse_matrix ----------------------------------------------------
+test_that("coerce_to_sparse_matrix() works", {
+  skip_if_not_installed("Matrix")
+
+  sparse_df <- lapply(1:10, function(x) sparse_double(x, x, length = 10))
+  names(sparse_df) <- letters[1:10]
+  sparse_df <- as.data.frame(sparse_df)
+
+  res <- coerce_to_sparse_matrix(sparse_df)
+  expect_s4_class(res, "dgCMatrix")
+  expect_identical(dim(res), c(10L, 10L))
+
+  exp <- Matrix::diag(1:10, 10, 10)
+  exp <- Matrix::Matrix(exp, sparse = TRUE)
+  exp <- as(exp, "generalMatrix")
+  exp <- as(exp, "CsparseMatrix")
+  colnames(exp) <- colnames(res)
+  rownames(exp) <- rownames(res)
+
+  expect_identical(res, exp)
+})
+
+test_that("coerce_to_sparse_matrix() errors on wrong input", {
+  skip_if_not_installed("Matrix")
+
+  expect_snapshot(
+    error = TRUE,
+    coerce_to_sparse_matrix(1:10)
+  )
+  expect_snapshot(
+    error = TRUE,
+    coerce_to_sparse_matrix(matrix(0, nrow = 10, ncol = 10))
+  )
+  expect_snapshot(
+    error = TRUE,
+    coerce_to_sparse_matrix(iris)
+  )
+})
+
+test_that("coerce_to_sparse_matrix() will divert for non-sparse data.frames", {
+  skip_if_not_installed("Matrix")
+
+  expect_identical(
+    coerce_to_sparse_matrix(mtcars),
+    Matrix::Matrix(as.matrix(mtcars), sparse = TRUE)
+  )
+})
+
+test_that("coerce_to_sparse_matrix() materializes non-zero defaulted columns", {
+  skip_if_not_installed("Matrix")
+  withr::local_options("sparsevctrs.verbose_materialize" = TRUE)
+
+  sparse_df <- lapply(1:10, function(x) sparse_double(x, x, length = 10))
+  names(sparse_df) <- letters[1:10]
+  sparse_df <- as.data.frame(sparse_df)
+  sparse_df$nonzero1 <- sparse_double(1, 1, 10, default = 10)
+  sparse_df$nonzero2 <- sparse_double(1, 1, 10, default = 20)
+  
+  expect_snapshot(
+    res <- coerce_to_sparse_matrix(sparse_df)
+  )
+
+  withr::local_options("sparsevctrs.verbose_materialize" = NULL)
+
+  expect_s4_class(res, "dgCMatrix")
+  expect_identical(dim(res), c(10L, 12L))
+
+  exp <- Matrix::diag(1:10, 10, 10)
+  exp <- Matrix::Matrix(exp, sparse = TRUE)
+  exp <- as(exp, "generalMatrix")
+  exp <- as(exp, "CsparseMatrix")
+
+  exp <- cbind(exp, sparse_df$nonzero1)
+  exp <- cbind(exp, sparse_df$nonzero2)
+
+  colnames(exp) <- colnames(res)
+  rownames(exp) <- rownames(res)
+
+  expect_identical(res, exp)
+})
+
+### coerce_to_sparse_data_frame ------------------------------------------------
+
+test_that("coerce_to_sparse_data_frame() works", {
+  skip_if_not_installed("Matrix")
+
+  sparse_mat <- Matrix::diag(1:10, 10, 10)
+  sparse_mat <- Matrix::Matrix(sparse_mat, sparse = TRUE)
+  sparse_mat <- as(sparse_mat, "generalMatrix")
+  sparse_mat <- as(sparse_mat, "CsparseMatrix")
+  colnames(sparse_mat) <- letters[1:10]
+  rownames(sparse_mat) <- 1:10
+
+  res <- coerce_to_sparse_data_frame(sparse_mat)
+
+  exp <- lapply(1:10, function(x) sparse_double(x, x, length = 10))
+  names(exp) <- letters[1:10]
+  exp <- as.data.frame(exp)
+
+  expect_identical(res, exp)
+})
+
+test_that("coerce_to_sparse_data_frame() works with non-dgCMatrix input", {
+  skip_if_not_installed("Matrix")
+
+  sparse_mat <- Matrix::diag(1:10, 10, 10)
+  sparse_mat <- Matrix::Matrix(sparse_mat, sparse = TRUE)
+  colnames(sparse_mat) <- letters[1:10]
+  rownames(sparse_mat) <- 1:10
+
+  res <- coerce_to_sparse_data_frame(sparse_mat)
+
+  exp <- lapply(1:10, function(x) sparse_double(x, x, length = 10))
+  names(exp) <- letters[1:10]
+  exp <- as.data.frame(exp)
+
+  expect_identical(res, exp)
+})
+
+test_that("coerce_to_sparse_data_frame() errors with no column names", {
+  skip_if_not_installed("Matrix")
+
+  sparse_mat <- Matrix::diag(1:10, 10, 10)
+  sparse_mat <- Matrix::Matrix(sparse_mat, sparse = TRUE)
+
+  expect_snapshot(
+    error = TRUE,
+    coerce_to_sparse_data_frame(sparse_mat)
+  )
+})
+
+test_that("coerce_to_sparse_data_frame() errors with wrong input", {
+  expect_snapshot(
+    error = TRUE,
+    coerce_to_sparse_data_frame(mtcars)
+  )
+  expect_snapshot(
+    error = TRUE,
+    coerce_to_sparse_data_frame(1:10)
+  )
+})
+
+### coerce_to_sparse_tibble ----------------------------------------------------
+
+test_that("coerce_to_sparse_tibble() works", {
+  skip_if_not_installed("Matrix")
+  skip_if_not_installed("tibble")
+
+  sparse_mat <- Matrix::diag(1:10, 10, 10)
+  sparse_mat <- Matrix::Matrix(sparse_mat, sparse = TRUE)
+  sparse_mat <- as(sparse_mat, "generalMatrix")
+  sparse_mat <- as(sparse_mat, "CsparseMatrix")
+  colnames(sparse_mat) <- letters[1:10]
+  rownames(sparse_mat) <- 1:10
+
+  res <- coerce_to_sparse_tibble(sparse_mat)
+
+  exp <- lapply(1:10, function(x) sparse_double(x, x, length = 10))
+  names(exp) <- letters[1:10]
+  exp <- tibble::as_tibble(exp)
+
+  expect_identical(res, exp)
+})
+
+test_that("coerce_to_sparse_tibble() works with non-dgCMatrix input", {
+  skip_if_not_installed("Matrix")
+  skip_if_not_installed("tibble")
+
+  sparse_mat <- Matrix::diag(1:10, 10, 10)
+  sparse_mat <- Matrix::Matrix(sparse_mat, sparse = TRUE)
+  colnames(sparse_mat) <- letters[1:10]
+  rownames(sparse_mat) <- 1:10
+
+  res <- coerce_to_sparse_tibble(sparse_mat)
+
+  exp <- lapply(1:10, function(x) sparse_double(x, x, length = 10))
+  names(exp) <- letters[1:10]
+  exp <- tibble::as_tibble(exp)
+
+  expect_identical(res, exp)
+})
+
+test_that("coerce_to_sparse_tibble() errors with no column names", {
+  skip_if_not_installed("Matrix")
+  skip_if_not_installed("tibble")
+
+  sparse_mat <- Matrix::diag(1:10, 10, 10)
+  sparse_mat <- Matrix::Matrix(sparse_mat, sparse = TRUE)
+
+  expect_snapshot(
+    error = TRUE,
+    coerce_to_sparse_tibble(sparse_mat)
+  )
+})
+
+test_that("coerce_to_sparse_tibble() errors with wrong input", {
+  expect_snapshot(
+    error = TRUE,
+    coerce_to_sparse_tibble(mtcars)
+  )
+  expect_snapshot(
+    error = TRUE,
+    coerce_to_sparse_tibble(1:10)
+  )
+})
\ No newline at end of file