diff --git a/src/osd/OSDMapMapping.h b/src/osd/OSDMapMapping.h
index 3f14cb8f0c7..e85e85dc6e0 100644
--- a/src/osd/OSDMapMapping.h
+++ b/src/osd/OSDMapMapping.h
@@ -230,8 +230,11 @@ private:
       int32_t *row = &table[row_size() * ps];
       row[0] = acting_primary;
       row[1] = up_primary;
-      row[2] = acting.size();
-      row[3] = up.size();
+      // these should always be <= the pool size, but just in case, avoid
+      // blowing out the array.  Note that our mapping is not completely
+      // accurate in this case--this is just to avoid crashing.
+      row[2] = std::min<int32_t>(acting.size(), size);
+      row[3] = std::min<int32_t>(up.size(), size);
       for (int i = 0; i < row[2]; ++i) {
 	row[4 + i] = acting[i];
       }