diff --git a/slinky/src/error.rs b/slinky/src/error.rs
index 24e6872..5262361 100644
--- a/slinky/src/error.rs
+++ b/slinky/src/error.rs
@@ -53,4 +53,10 @@ pub enum SlinkyError {
         section: Cow<'static, str>,
         segment: Cow<'static, str>,
     },
+
+    #[error("Segment '{segment}' references undefined vram class '{vram_class}'")]
+    MissingVramClassForSegment {
+        segment: Cow<'static, str>,
+        vram_class: Cow<'static, str>,
+    },
 }
diff --git a/slinky/src/linker_writer.rs b/slinky/src/linker_writer.rs
index 225c66a..de91559 100644
--- a/slinky/src/linker_writer.rs
+++ b/slinky/src/linker_writer.rs
@@ -1,6 +1,7 @@
 /* SPDX-FileCopyrightText: © 2024 decompals */
 /* SPDX-License-Identifier: MIT */
 
+use std::borrow::Cow;
 use std::io::Write;
 
 use crate::{
@@ -518,8 +519,15 @@ impl LinkerWriter<'_> {
         let main_seg_sym_size: String = style.segment_vram_size(&segment.name);
 
         if let Some(vram_class_name) = &segment.vram_class {
-            // TODO: return Err if vram class is not present instead of unwrapping
-            let vram_class = self.vram_classes.get_mut(vram_class_name).unwrap();
+            let vram_class = match self.vram_classes.get_mut(vram_class_name) {
+                Some(vc) => vc,
+                None => {
+                    return Err(SlinkyError::MissingVramClassForSegment {
+                        segment: Cow::from(segment.name.clone()),
+                        vram_class: Cow::from(vram_class_name.clone()),
+                    })
+                }
+            };
 
             if !vram_class.emitted {
                 let vram_class_sym = style.vram_class_start(vram_class_name);