diff --git a/src/mesa/main/image.c b/src/mesa/main/image.c
index 65b7a62b528..a945ba2e146 100644
--- a/src/mesa/main/image.c
+++ b/src/mesa/main/image.c
@@ -368,7 +368,8 @@ _mesa_bytes_per_pixel( GLenum format, GLenum type )
  * otherwise.
  */
 GLboolean
-_mesa_is_legal_format_and_type( struct gl_context *ctx, GLenum format, GLenum type )
+_mesa_is_legal_format_and_type(const struct gl_context *ctx,
+                               GLenum format, GLenum type)
 {
    switch (format) {
       case GL_COLOR_INDEX:
diff --git a/src/mesa/main/image.h b/src/mesa/main/image.h
index b40717223e1..023284a9fbc 100644
--- a/src/mesa/main/image.h
+++ b/src/mesa/main/image.h
@@ -52,7 +52,8 @@ extern GLint
 _mesa_bytes_per_pixel( GLenum format, GLenum type );
 
 extern GLboolean
-_mesa_is_legal_format_and_type( struct gl_context *ctx, GLenum format, GLenum type );
+_mesa_is_legal_format_and_type(const struct gl_context *ctx,
+                               GLenum format, GLenum type);
 
 extern GLboolean
 _mesa_is_color_format(GLenum format);