diff --git a/src/mesa/main/texenvprogram.c b/src/mesa/main/texenvprogram.c
index 6aa89d1..fa4c79e 100644
--- a/src/mesa/main/texenvprogram.c
+++ b/src/mesa/main/texenvprogram.c
@@ -488,6 +488,7 @@ static GLuint make_state_key( GLcontext *ctx,  struct state_key *key )
 
    /* _NEW_BUFFERS */
    key->num_draw_buffers = ctx->DrawBuffer->_NumColorDrawBuffers;
+   assert(key->num_draw_buffers > 0);
 
    key->inputs_available = (inputs_available & inputs_referenced);