diff --git a/nasmlib.c b/nasmlib.c
index 34dbbcf5..8796879e 100644
--- a/nasmlib.c
+++ b/nasmlib.c
@@ -597,7 +597,7 @@ int src_get(int32_t *xline, char **xname)
     return 0;
 }
 
-char *nasm_strcat(char *one, char *two)
+char *nasm_strcat(const char *one, const char *two)
 {
     char *rslt;
     int l1 = strlen(one);
diff --git a/nasmlib.h b/nasmlib.h
index cb802b13..d9015207 100644
--- a/nasmlib.h
+++ b/nasmlib.h
@@ -321,7 +321,7 @@ int32_t src_get_linnum(void);
  */
 int src_get(int32_t *xline, char **xname);
 
-char *nasm_strcat(char *one, char *two);
+char *nasm_strcat(const char *one, const char *two);
 
 void null_debug_routine(const char *directive, const char *params);
 extern struct dfmt null_debug_form;