diff --git a/core/src/main/java/org/apache/spark/unsafe/map/BytesToBytesMap.java b/core/src/main/java/org/apache/spark/unsafe/map/BytesToBytesMap.java
index 7590b72112c5..7bdd89488d11 100644
--- a/core/src/main/java/org/apache/spark/unsafe/map/BytesToBytesMap.java
+++ b/core/src/main/java/org/apache/spark/unsafe/map/BytesToBytesMap.java
@@ -694,7 +694,10 @@ public boolean append(Object kbase, long koff, int klen, Object vbase, long voff
       assert (vlen % 8 == 0);
       assert (longArray != null);
 
-      if (numKeys == MAX_CAPACITY
+      // We should not increase number of keys to be MAX_CAPACITY. The usage pattern of this map is
+      // lookup + append. If we append key until the number of keys to be MAX_CAPACITY, next time
+      // the call of lookup will hang forever because it cannot find an empty slot.
+      if (numKeys == MAX_CAPACITY - 1
         // The map could be reused from last spill (because of no enough memory to grow),
         // then we don't try to grow again if hit the `growthThreshold`.
         || !canGrowArray && numKeys >= growthThreshold) {