diff --git a/common.h b/common.h
index ee537a8..14fdfe9 100644
--- a/common.h
+++ b/common.h
@@ -83,6 +83,11 @@
 	return ret;
 }
 
+static inline bool is_power_of_2(unsigned val)
+{
+	return __builtin_popcount(val) == 1;
+}
+
 char *get_dev_mtd(void);
 
 #endif /* COMMON_H */
diff --git a/mboxd.c b/mboxd.c
index 0ebba56..1307fe6 100644
--- a/mboxd.c
+++ b/mboxd.c
@@ -241,6 +241,10 @@
 				fprintf(stderr, "Unparseable window size\n");
 				return false;
 			}
+			if (!is_power_of_2(context->windows.default_size)) {
+				fprintf(stderr, "Window size not power of 2\n");
+				return false;
+			}
 			break;
 		case 'v':
 			verbosity++;