diff --git a/src/nfa/limex_dump.cpp b/src/nfa/limex_dump.cpp
index c52adc46..8b6b7015 100644
--- a/src/nfa/limex_dump.cpp
+++ b/src/nfa/limex_dump.cpp
@@ -82,7 +82,7 @@ void dumpMask(FILE *f, const char *name, const u8 *mask, u32 mask_bits) {
 
 template<typename mask_t>
 static
-u32 rank_in_mask(mask_t mask, u32 bit) {
+u32 rank_in_mask(const mask_t &mask, u32 bit) {
     assert(bit < 8 * sizeof(mask));
 
     u32 chunks[sizeof(mask)/sizeof(u32)];