diff --git a/src/components/gfx/font.rs b/src/components/gfx/font.rs
index 9278cc11041..994ee0d91de 100644
--- a/src/components/gfx/font.rs
+++ b/src/components/gfx/font.rs
@@ -49,7 +49,7 @@ pub trait FontHandleMethods {
     fn clone_with_style(&self, fctx: &FontContextHandle, style: &UsedFontStyle)
                      -> Result<FontHandle, ()>;
     fn glyph_index(&self, codepoint: char) -> Option<GlyphIndex>;
-    fn glyph_h_advance(&mut self, GlyphIndex) -> Option<FractionalPixel>;
+    fn glyph_h_advance(&self, GlyphIndex) -> Option<FractionalPixel>;
     fn get_metrics(&self) -> FontMetrics;
     fn get_table_for_tag(&self, FontTableTag) -> Option<FontTable>;
 }
diff --git a/src/components/gfx/platform/android/font.rs b/src/components/gfx/platform/android/font.rs
index a9e20379757..b8b08cc9599 100644
--- a/src/components/gfx/platform/android/font.rs
+++ b/src/components/gfx/platform/android/font.rs
@@ -198,7 +198,7 @@ impl FontHandleMethods for FontHandle {
     }
 
     #[fixed_stack_segment]
-    fn glyph_h_advance(&mut self,
+    fn glyph_h_advance(&self,
                            glyph: GlyphIndex) -> Option<FractionalPixel> {
         assert!(self.face.is_not_null());
         unsafe {
diff --git a/src/components/gfx/platform/linux/font.rs b/src/components/gfx/platform/linux/font.rs
index a9e20379757..b8b08cc9599 100644
--- a/src/components/gfx/platform/linux/font.rs
+++ b/src/components/gfx/platform/linux/font.rs
@@ -198,7 +198,7 @@ impl FontHandleMethods for FontHandle {
     }
 
     #[fixed_stack_segment]
-    fn glyph_h_advance(&mut self,
+    fn glyph_h_advance(&self,
                            glyph: GlyphIndex) -> Option<FractionalPixel> {
         assert!(self.face.is_not_null());
         unsafe {
diff --git a/src/components/gfx/platform/macos/font.rs b/src/components/gfx/platform/macos/font.rs
index f38d7600bb9..08978c5d063 100644
--- a/src/components/gfx/platform/macos/font.rs
+++ b/src/components/gfx/platform/macos/font.rs
@@ -147,7 +147,7 @@ impl FontHandleMethods for FontHandle {
         return Some(glyphs[0] as GlyphIndex);
     }
 
-    fn glyph_h_advance(&mut self, glyph: GlyphIndex) -> Option<FractionalPixel> {
+    fn glyph_h_advance(&self, glyph: GlyphIndex) -> Option<FractionalPixel> {
         let glyphs = [glyph as CGGlyph];
         let advance = self.ctfont.get_advances_for_glyphs(kCTFontDefaultOrientation,
                                                           &glyphs[0],