diff --git a/lib/std/heap/ArenaAllocator.zig b/lib/std/heap/ArenaAllocator.zig
index 3532bd07af0daec1bf166529bbb5562c5f3b1028..8888ebdbee3573b997e018f007d38693819b0a9d 100644
--- a/lib/std/heap/ArenaAllocator.zig
+++ b/lib/std/heap/ArenaAllocator.zig
@@ -572,7 +572,8 @@ fn resize(ctx: *anyopaque, memory: []u8, alignment: Alignment, new_len: usize, r
         new_end_index,
         .monotonic,
         .monotonic,
-    );
+    ) or
+        new_len <= memory.len; // Shrinking allocations should always succeed.
 }
 
 fn remap(ctx: *anyopaque, memory: []u8, alignment: Alignment, new_len: usize, ret_addr: usize) ?[*]u8 {
diff --git a/lib/std/heap/FixedBufferAllocator.zig b/lib/std/heap/FixedBufferAllocator.zig
index 1303bd35abbfcd378eed42553f046772dd40cb9d..e15875948a977b6dff2193c4f44ec3399cc48aa1 100644
--- a/lib/std/heap/FixedBufferAllocator.zig
+++ b/lib/std/heap/FixedBufferAllocator.zig
@@ -172,7 +172,8 @@ fn threadSafeResize(ctx: *anyopaque, memory: []u8, alignment: mem.Alignment, new
         new_end_index,
         .monotonic,
         .monotonic,
-    );
+    ) or
+        new_len <= memory.len; // Shrinking allocations should always succeed.
 }
 
 fn threadSafeRemap(ctx: *anyopaque, memory: []u8, alignment: mem.Alignment, new_len: usize, ret_addr: usize) ?[*]u8 {