diff --git a/externals/Makefile b/externals/Makefile
index 982b0e5cc436cb16878ba746a455172445749d21..3cefc37fc5ed505805856f0ce57b20172a2eeba5 100644
--- a/externals/Makefile
+++ b/externals/Makefile
@@ -542,7 +542,7 @@ fftease_clean:
 #
 flatgui:
 	make -C $(externals_src)/footils/knob CFLAGS="$(CFLAGS)" \
-		PD_PATH=$(pd_src) pdbinpath=$(pd_src/src) \
+		PD_PATH=$(pd_src) pdbinpath=$(pd_src)/src \
 		PD_INCLUDE=$(pd_src)/src
 
 flatgui_install: