diff --git a/src/intermission/intermission_parse.cpp b/src/intermission/intermission_parse.cpp
index f9ae2fd5e..e227512f4 100644
--- a/src/intermission/intermission_parse.cpp
+++ b/src/intermission/intermission_parse.cpp
@@ -34,6 +34,7 @@
 */
 
 
+#include <ctype.h>
 #include "intermission/intermission.h"
 #include "g_level.h"
 #include "w_wad.h"
diff --git a/src/r_data/voxels.cpp b/src/r_data/voxels.cpp
index ae16eac1a..02a63d4fe 100644
--- a/src/r_data/voxels.cpp
+++ b/src/r_data/voxels.cpp
@@ -35,6 +35,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <algorithm>
+#include <ctype.h>
 
 #include "templates.h"
 #include "doomdef.h"
diff --git a/src/resourcefiles/file_wad.cpp b/src/resourcefiles/file_wad.cpp
index dca1188c9..4b62bbeec 100644
--- a/src/resourcefiles/file_wad.cpp
+++ b/src/resourcefiles/file_wad.cpp
@@ -33,6 +33,7 @@
 **
 */
 
+#include <ctype.h>
 #include "resourcefile.h"
 #include "cmdlib.h"
 #include "templates.h"