diff --git a/funcs/Makefile b/funcs/Makefile
index c80ebcbe..ee4c6493 100644
--- a/funcs/Makefile
+++ b/funcs/Makefile
@@ -21,7 +21,7 @@ t:	t.c $(DEPS) ../libfloatimg.a tests.o
 		-lz -lm -o $@
 
 tests.o:	tests.c tests.h $(DEPS)
-	gcc $(COPT) -c $<
+	gcc $(COPT) -I/usr/include/netpbm -c $<
 
 #---------------------------------------------------------------
 #	upper-level functions