diff --git a/crates/uv-workspace/src/pyproject.rs b/crates/uv-workspace/src/pyproject.rs
index cf3a964293b5..9c5072647391 100644
--- a/crates/uv-workspace/src/pyproject.rs
+++ b/crates/uv-workspace/src/pyproject.rs
@@ -799,23 +799,23 @@ impl TryFrom<SourcesWire> for Sources {
                     .zip(sources.iter().skip(1).map(Source::marker))
                 {
                     if !lhs.is_disjoint(&rhs) {
+                        let Some(left) = lhs.contents().map(|contents| contents.to_string()) else {
+                            return Err(SourceError::MissingMarkers);
+                        };
+
+                        let Some(right) = rhs.contents().map(|contents| contents.to_string())
+                        else {
+                            return Err(SourceError::MissingMarkers);
+                        };
+
                         let mut hint = lhs.negate();
                         hint.and(rhs.clone());
-
-                        let lhs = lhs
-                            .contents()
-                            .map(|contents| contents.to_string())
-                            .unwrap_or_else(|| "true".to_string());
-                        let rhs = rhs
-                            .contents()
-                            .map(|contents| contents.to_string())
-                            .unwrap_or_else(|| "true".to_string());
                         let hint = hint
                             .contents()
                             .map(|contents| contents.to_string())
                             .unwrap_or_else(|| "true".to_string());
 
-                        return Err(SourceError::OverlappingMarkers(lhs, rhs, hint));
+                        return Err(SourceError::OverlappingMarkers(left, right, hint));
                     }
                 }
 
@@ -1231,6 +1231,8 @@ pub enum SourceError {
     NonUtf8Path(PathBuf),
     #[error("Source markers must be disjoint, but the following markers overlap: `{0}` and `{1}`.\n\n{hint}{colon} replace `{1}` with `{2}`.", hint = "hint".bold().cyan(), colon = ":".bold())]
     OverlappingMarkers(String, String, String),
+    #[error("When multiple sources are provided, each source must include a platform markers (e.g., `marker = \"sys_platform == 'linux'\"`)")]
+    MissingMarkers,
     #[error("Must provide at least one source")]
     EmptySources,
 }
diff --git a/crates/uv/tests/it/lock.rs b/crates/uv/tests/it/lock.rs
index b640383ed1ab..cf4f811d7efe 100644
--- a/crates/uv/tests/it/lock.rs
+++ b/crates/uv/tests/it/lock.rs
@@ -18036,6 +18036,44 @@ fn lock_multiple_sources_conflict() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn lock_multiple_sources_no_marker() -> Result<()> {
+    let context = TestContext::new("3.12");
+
+    let pyproject_toml = context.temp_dir.child("pyproject.toml");
+    pyproject_toml.write_str(
+        r#"
+        [project]
+        name = "project"
+        version = "0.1.0"
+        requires-python = ">=3.12"
+        dependencies = ["iniconfig"]
+
+        [tool.uv.sources]
+        iniconfig = [
+            { url = "https://files.pythonhosted.org/packages/ef/a6/62565a6e1cf69e10f5727360368e451d4b7f58beeac6173dc9db836a5b46/iniconfig-2.0.0-py3-none-any.whl" },
+            { url = "https://files.pythonhosted.org/packages/d7/4b/cbd8e699e64a6f16ca3a8220661b5f83792b3017d0f79807cb8708d33913/iniconfig-2.0.0.tar.gz" },
+        ]
+        "#,
+    )?;
+
+    uv_snapshot!(context.filters(), context.lock(), @r###"
+    success: false
+    exit_code: 2
+    ----- stdout -----
+
+    ----- stderr -----
+    error: Failed to parse: `pyproject.toml`
+      Caused by: TOML parse error at line 9, column 21
+      |
+    9 |         iniconfig = [
+      |                     ^
+    When multiple sources are provided, each source must include a platform markers (e.g., `marker = "sys_platform == 'linux'"`)
+    "###);
+
+    Ok(())
+}
+
 #[test]
 fn lock_multiple_sources_index() -> Result<()> {
     let context = TestContext::new("3.12");