diff --git a/Ibm1130/ibm1130_cpu.c b/Ibm1130/ibm1130_cpu.c
index aba74fb6a..f82b610a8 100644
--- a/Ibm1130/ibm1130_cpu.c
+++ b/Ibm1130/ibm1130_cpu.c
@@ -653,7 +653,7 @@ t_stat sim_instr (void)
 
             eaddr = word2;                  /* assume standard addressing & compute effective address */
             if (TAG)                        /* if indexed */
-                eaddr += ReadIndex(TAG);    /* add index register value */
+		eaddr = (eaddr + ReadIndex(TAG)) & 0xFFFF;	/* add index register value */
             if (INDIR)                      /* if indirect addressing */
                 eaddr = ReadW(eaddr);       /* pick up referenced address */