diff --git a/interpolate.c b/interpolate.c
index 62701d843..5d9d1889f 100644
--- a/interpolate.c
+++ b/interpolate.c
@@ -8,10 +8,10 @@
 #include "interpolate.h"
 
 
-void interp_set_entry(struct interp *table, int slot, char *value)
+void interp_set_entry(struct interp *table, int slot, const char *value)
 {
 	char *oldval = table[slot].value;
-	char *newval = value;
+	char *newval = NULL;
 
 	if (oldval)
 		free(oldval);
diff --git a/interpolate.h b/interpolate.h
index a55fb8e07..190a180b5 100644
--- a/interpolate.h
+++ b/interpolate.h
@@ -16,7 +16,7 @@ struct interp {
 	char *value;
 };
 
-extern void interp_set_entry(struct interp *table, int slot, char *value);
+extern void interp_set_entry(struct interp *table, int slot, const char *value);
 extern void interp_clear_table(struct interp *table, int ninterps);
 
 extern int interpolate(char *result, int reslen,