diff --git a/src/stralloc.h b/src/stralloc.h
index 4a39cb38d4f2a41d1907e9b3a17aa9d633e1a785..e2e2d84e56d3be9f14fcbe9c6d57b031eae79c64 100644
--- a/src/stralloc.h
+++ b/src/stralloc.h
@@ -143,7 +143,7 @@ static INLINE p_wchar2 index_shared_string(const struct pike_string *s,  ptrdiff
 }
 #else
 PMOD_EXPORT p_wchar2 generic_extract (const void *str, int size, ptrdiff_t pos);
-PMOD_EXPORT p_wchar2 index_shared_string(struct pike_string *s, ptrdiff_t pos);
+PMOD_EXPORT p_wchar2 index_shared_string(const struct pike_string *s, ptrdiff_t pos);
 #endif
 
 #define INDEX_CHARP(PTR,IND,SHIFT) generic_extract(PTR,SHIFT,IND)