diff --git a/Source/PINMemoryCache.m b/Source/PINMemoryCache.m
index 1442da0..93edd0c 100644
--- a/Source/PINMemoryCache.m
+++ b/Source/PINMemoryCache.m
@@ -175,6 +175,10 @@ - (void)removeObjectAndExecuteBlocksForKey:(NSString *)key
         PINCacheObjectBlock didRemoveObjectBlock = _didRemoveObjectBlock;
     [self unlock];
 
+    if (object == nil) {
+        return;
+    }
+
     if (willRemoveObjectBlock)
         willRemoveObjectBlock(self, key, object);
 
@@ -239,13 +243,18 @@ - (void)trimToCostLimit:(NSUInteger)limit
     
     [self lock];
         totalCost = _totalCost;
-        NSArray *keysSortedByCost = [_costs keysSortedByValueUsingSelector:@selector(compare:)];
     [self unlock];
     
     if (totalCost <= limit) {
         return;
     }
 
+    [self lock];
+        NSDictionary *costs = [_costs copy];
+    [self unlock];
+
+    NSArray *keysSortedByCost = [costs keysSortedByValueUsingSelector:@selector(compare:)];
+
     for (NSString *key in [keysSortedByCost reverseObjectEnumerator]) { // costliest objects first
         [self removeObjectAndExecuteBlocksForKey:key];
 
@@ -272,9 +281,11 @@ - (void)trimToCostLimitByDate:(NSUInteger)limit
         return;
     
     [self lock];
-        NSArray *keysSortedByAccessDate = [_accessDates keysSortedByValueUsingSelector:@selector(compare:)];
+        NSDictionary *accessDates = [_accessDates copy];
     [self unlock];
 
+    NSArray *keysSortedByAccessDate = [accessDates keysSortedByValueUsingSelector:@selector(compare:)];
+
     for (NSString *key in keysSortedByAccessDate) { // oldest objects first
         [self removeObjectAndExecuteBlocksForKey:key];