--- a/src/gf.cc
+++ b/src/gf.cc
@@ -2251,7 +2251,7 @@
       RowVector found (n, 0);
       found(0) = 1;
       galois c (1, m, 0, m);
-      c(0, 0) == c.index_of (1);
+      c(0, 0) = c.index_of (1);
       Array<int> cs (dim_vector (1, 1), 1);
 
       int nc = 1;