diff --git a/src/gtkutil.c b/src/gtkutil.c
index 6308c38f164..5e7cf3d2114 100644
--- a/src/gtkutil.c
+++ b/src/gtkutil.c
@@ -22,6 +22,7 @@ along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.  */
 #ifdef USE_GTK
 #include <float.h>
 #include <stdio.h>
+#include <stdlib.h>
 
 #include <c-ctype.h>