diff --git a/src/w_merge.c b/src/w_merge.c
index 2c525a24..56ba4045 100644
--- a/src/w_merge.c
+++ b/src/w_merge.c
@@ -567,7 +567,7 @@ void W_PrintDirectory(void)
 
 // Merge in a file by name
 
-void W_MergeFile(char *filename)
+void W_MergeFile(const char *filename)
 {
     int old_numlumps;
 
diff --git a/src/w_merge.h b/src/w_merge.h
index c8ecc69f..a7803a34 100644
--- a/src/w_merge.h
+++ b/src/w_merge.h
@@ -26,7 +26,7 @@
 
 // Add a new WAD and merge it into the main directory
 
-void W_MergeFile(char *filename);
+void W_MergeFile(const char *filename);
 
 // NWT-style merging