diff --git a/dlls/comctl32/comctl32undoc.c b/dlls/comctl32/comctl32undoc.c
index ce7409940a5..416d35d84c9 100644
--- a/dlls/comctl32/comctl32undoc.c
+++ b/dlls/comctl32/comctl32undoc.c
@@ -843,7 +843,7 @@ DSA_SetItem (const HDSA hdsa, INT nIndex, LPVOID pSrc)
 	else {
 	    /* resize the block of memory */
 	    nNewItems =
-		hdsa->nGrow * ((INT)((nIndex - 1) / hdsa->nGrow) + 1);
+		hdsa->nGrow * ((INT)(((nIndex + 1) - 1) / hdsa->nGrow) + 1);
 	    nSize = hdsa->nItemSize * nNewItems;
 
 	    lpTemp = (LPVOID)COMCTL32_ReAlloc (hdsa->pData, nSize);
@@ -1349,7 +1349,7 @@ DPA_SetPtr (const HDPA hdpa, INT i, LPVOID p)
 	else {
 	    /* resize the block of memory */
 	    INT nNewItems =
-		hdpa->nGrow * ((INT)((i - 1) / hdpa->nGrow) + 1);
+		hdpa->nGrow * ((INT)(((i+1) - 1) / hdpa->nGrow) + 1);
 	    INT nSize = nNewItems * sizeof(LPVOID);
 
 	    lpTemp = (LPVOID*)HeapReAlloc (hdpa->hHeap, HEAP_ZERO_MEMORY,