diff --git a/src/mzscheme/sgc/collect.inc b/src/mzscheme/sgc/collect.inc
index 81acafdf25..e4ee61f0c6 100644
--- a/src/mzscheme/sgc/collect.inc
+++ b/src/mzscheme/sgc/collect.inc
@@ -164,7 +164,7 @@ static void COLLECT(OFFSET_ARG)
 	    if ((p >= bstart) && (p < block->top)) {
 	      int size = block->size;
 	      int pos = block->positions[(p - bstart) >> LOG_PTR_SIZE];
-	      int start = bstart + pos * size;
+	      unsigned long start = bstart + pos * size;
 	    
 	      MARK_STATISTIC(num_blockallocs_stat++);
 
diff --git a/src/mzscheme/sgc/sgc.c b/src/mzscheme/sgc/sgc.c
index 426fe87842..f4566a9298 100644
--- a/src/mzscheme/sgc/sgc.c
+++ b/src/mzscheme/sgc/sgc.c
@@ -822,7 +822,7 @@ static void *malloc_plain_sector(int count);
 inline static SectorPage **create_sector_pagetables(unsigned long p) {
   unsigned long pos;
   SectorPage ***sector_pagetabless, **sector_pagetables;
-  pos = (unsigned long)p >> 48;
+  pos = ((unsigned long)p >> 48) & ((1 << 16) - 1);
   sector_pagetabless = sector_pagetablesss[pos];
   if (!sector_pagetabless) {
     int c = (sizeof(SectorPage **) << 16) >> LOG_SECTOR_SEGMENT_SIZE;
@@ -848,7 +848,7 @@ inline static SectorPage **create_sector_pagetables(unsigned long p) {
 inline static SectorPage **get_sector_pagetables(unsigned long p) {
   unsigned long pos;
   SectorPage ***sector_pagetabless;
-  pos = (unsigned long)p >> 48;
+  pos = ((unsigned long)p >> 48) & ((1 << 16) - 1);
   sector_pagetabless = sector_pagetablesss[pos];
   if (!sector_pagetabless)
     return NULL;