diff --git a/src/mesa/main/buffers.c b/src/mesa/main/buffers.c
index 2bdbf41..6c846cb 100644
--- a/src/mesa/main/buffers.c
+++ b/src/mesa/main/buffers.c
@@ -623,7 +623,9 @@ _mesa_ReadBuffer(GLenum buffer)
 
    if (buffer == GL_NONE) {
       /* This is legal--it means that no buffer should be bound for reading. */
-      srcBuffer = -1;
+      _mesa_error(ctx, GL_INVALID_ENUM,
+                           "glReadBuffer(buffer=0x%x)", buffer);
+	 return;
    }
    else {
       /* general case / window-system framebuffer */