diff --git a/src/map/if/acd/ac_decomposition.hpp b/src/map/if/acd/ac_decomposition.hpp
index 8d5ddb5c43..c6df54662e 100644
--- a/src/map/if/acd/ac_decomposition.hpp
+++ b/src/map/if/acd/ac_decomposition.hpp
@@ -482,6 +482,11 @@ class ac_decomposition_impl
     for ( uint32_t i = 0; i < num_vars; ++i )
     {
       pComb[i] = pInvPerm[i] = i;
+      /* bestPerm is written only when some combination beats the initial
+       * best_cost.  When none does, the loop below still evaluates
+       * permutations[bestPerm[i]], which reads uninitialised stack and then
+       * indexes permutations[] with it.  Seed the identity permutation. */
+      bestPerm[i] = i;
     }
 
     /* early bail-out conditions */