diff --git a/Makefile b/Makefile
index dbe39d8..c9443f4 100644
--- a/Makefile
+++ b/Makefile
@@ -8,7 +8,7 @@ CPPFLAGS := -I.
 CFLAGS   := -std=c99 -Wall
 LDFLAGS  := -lrt -lm
 
-CFLAGS += $(MODS:%=-DHAVE_MOD_%)
+CFLAGS += $(shell echo $(MODS:%=-DHAVE_MOD_%) | tr 'a-z' 'A-Z')
 
 OBJS := rosella.o gc.o builtin.o interp.o reader.o $(MODS:%=mods/mod_%.o)
 DEPS := $(OBJS:%.o=%.d)
diff --git a/rosella.c b/rosella.c
index ba2778d..02c4463 100644
--- a/rosella.c
+++ b/rosella.c
@@ -12,7 +12,7 @@
 #include "interp.h"
 #include "reader.h"
 
-#ifdef HAVE_MOD_io
+#ifdef HAVE_MOD_IO
 #  include "mods/mod_io.h"
 #endif
 
@@ -39,7 +39,7 @@ int main(int argc, char **argv)
   builtin_init();
   interpreter_init();
 
-#ifdef HAVE_MOD_io
+#ifdef HAVE_MOD_IO
   mod_io_init();
 #endif