diff --git a/gfx/thebes/gfxMacPlatformFontList.mm b/gfx/thebes/gfxMacPlatformFontList.mm
index 2623fbfb4436..955cf432d983 100644
--- a/gfx/thebes/gfxMacPlatformFontList.mm
+++ b/gfx/thebes/gfxMacPlatformFontList.mm
@@ -334,9 +334,11 @@ MacOSFontEntry::GetVariationAxes(nsTArray<gfxFontVariationAxis>& aVariationAxes)
                             kCTFontVariationAxisNameKey);
             if (name) {
                 CFIndex len = CFStringGetLength(name);
-                axis.mName.SetLength(len);
+                nsAutoString nameStr;
+                nameStr.SetLength(len);
                 CFStringGetCharacters(name, CFRangeMake(0, len),
-                                      (UniChar*)axis.mName.BeginWriting());
+                                      (UniChar*)nameStr.BeginWriting());
+                AppendUTF16toUTF8(nameStr, axis.mName);
             }
             axis.mTag = (uint32_t)tag;
             axis.mMinValue = minValue;