diff --git a/components/gfx/text/glyph.rs b/components/gfx/text/glyph.rs
index 5ebd33916e8..c5f803ca673 100644
--- a/components/gfx/text/glyph.rs
+++ b/components/gfx/text/glyph.rs
@@ -156,8 +156,11 @@ fn is_simple_glyph_id(id: GlyphId) -> bool {
 }
 
 fn is_simple_advance(advance: Au) -> bool {
-    let unsigned_au = advance.to_u32().unwrap();
-    (unsigned_au & (GLYPH_ADVANCE_MASK >> GLYPH_ADVANCE_SHIFT as uint)) == unsigned_au
+    match advance.to_u32() {
+        Some(unsigned_au) =>
+            (unsigned_au & (GLYPH_ADVANCE_MASK >> GLYPH_ADVANCE_SHIFT as uint)) == unsigned_au,
+        None => false
+    }
 }
 
 type DetailedGlyphCount = u16;
diff --git a/tests/ref/basic.list b/tests/ref/basic.list
index c66e6750ad5..3bbfc9c52d3 100644
--- a/tests/ref/basic.list
+++ b/tests/ref/basic.list
@@ -102,6 +102,7 @@ flaky_cpu == append_style_a.html append_style_b.html
 == floated_generated_content_a.html floated_generated_content_b.html
 == floated_list_item_a.html floated_list_item_ref.html
 == floated_table_with_margin_a.html floated_table_with_margin_ref.html
+== font_advance.html font_advance_ref.html
 == font_size.html font_size_ref.html
 == font_style.html font_style_ref.html
 == height_compute_reset.html height_compute.html
diff --git a/tests/ref/font_advance.html b/tests/ref/font_advance.html
new file mode 100644
index 00000000000..91d871a656e
--- /dev/null
+++ b/tests/ref/font_advance.html
@@ -0,0 +1,2 @@
+<!-- Not a reftest, but a crashtest for #4614 -->
+a<div id="test" style="font-size: 100000px">Hello world</div>
diff --git a/tests/ref/font_advance_ref.html b/tests/ref/font_advance_ref.html
new file mode 100644
index 00000000000..91d871a656e
--- /dev/null
+++ b/tests/ref/font_advance_ref.html
@@ -0,0 +1,2 @@
+<!-- Not a reftest, but a crashtest for #4614 -->
+a<div id="test" style="font-size: 100000px">Hello world</div>