--- a/content/canvas/src/WebGLContextGL.cpp
+++ b/content/canvas/src/WebGLContextGL.cpp
@@ -130,17 +130,17 @@ NS_IMETHODIMP WebGLContext::name(t1 a1, 
 /* void GlActiveTexture (in GLenum texture); */
 NS_IMETHODIMP
 WebGLContext::ActiveTexture(WebGLenum texture)
 {
     if (!IsContextStable())
         return NS_OK;
 
     if (texture < LOCAL_GL_TEXTURE0 ||
-        texture >= LOCAL_GL_TEXTURE0 + mGLMaxTextureUnits)
+        texture >= LOCAL_GL_TEXTURE0 + PRUint32(mGLMaxTextureUnits))
     {
         return ErrorInvalidEnum(
             "ActiveTexture: texture unit %d out of range. "
             "Accepted values range from TEXTURE0 to TEXTURE0 + %d. "
             "Notice that TEXTURE0 != 0.",
             texture, mGLMaxTextureUnits);
     }