diff --git a/src/chmatch.c b/src/chmatch.c
index 8dba9308e..4406373c4 100644
--- a/src/chmatch.c
+++ b/src/chmatch.c
@@ -95,7 +95,7 @@ static SEXP chmatchMain(SEXP x, SEXP table, int nomatch, bool chin, bool chmatch
     // For example: A,B,C,B,D,E,A,A   =>   A(TL=1),B(2),C(3),D(4),E(5)   =>   dupMap    1  2  3  5  6 | 8  7  4
     //                                                                        dupLink   7  8          |    6     (blank=0)
     int *counts = (int *)calloc(nuniq, sizeof(int));
-    int mapsize = tablelen+nuniq; // lto compilation warning #5760
+    unsigned int mapsize = tablelen+nuniq; // lto compilation warning #5760
     int *map =    (int *)calloc(mapsize, sizeof(int));  // +nuniq to store a 0 at the end of each group
     if (!counts || !map) {
       // # nocov start