diff --git a/src/aig/gia/giaTsim.c b/src/aig/gia/giaTsim.c
index 7f93c5426..1920cd6d3 100644
--- a/src/aig/gia/giaTsim.c
+++ b/src/aig/gia/giaTsim.c
@@ -84,9 +84,11 @@ Gia_ManTer_t * Gia_ManTerCreate( Gia_Man_t * pAig )
     p = ABC_CALLOC( Gia_ManTer_t, 1 );
     p->pAig   = Gia_ManFront( pAig );
     p->nIters = 300;
-    p->pDataSim    = ABC_ALLOC( unsigned, Abc_BitWordNum(2*p->pAig->nFront) );
-    p->pDataSimCis = ABC_ALLOC( unsigned, Abc_BitWordNum(2*Gia_ManCiNum(p->pAig)) );
-    p->pDataSimCos = ABC_ALLOC( unsigned, Abc_BitWordNum(2*Gia_ManCoNum(p->pAig)) );
+    // these buffers are accessed through XOR-based setters that read the current word first,
+    // so they must be zero-initialized to avoid touching undefined data on the first update
+    p->pDataSim    = ABC_CALLOC( unsigned, Abc_BitWordNum(2*p->pAig->nFront) );
+    p->pDataSimCis = ABC_CALLOC( unsigned, Abc_BitWordNum(2*Gia_ManCiNum(p->pAig)) );
+    p->pDataSimCos = ABC_CALLOC( unsigned, Abc_BitWordNum(2*Gia_ManCoNum(p->pAig)) );
     // allocate storage for terminary states
     p->nStateWords = Abc_BitWordNum( 2*Gia_ManRegNum(pAig) );
     p->vStates  = Vec_PtrAlloc( 1000 );
@@ -754,4 +756,3 @@ Gia_Man_t * Gia_ManReduceConst( Gia_Man_t * pAig, int fVerbose )
 
 
 ABC_NAMESPACE_IMPL_END
-