diff --git a/buffer/bs_capacitycheck.c b/buffer/bs_capacitycheck.c
index dfc2c84..5c4d9da 100644
--- a/buffer/bs_capacitycheck.c
+++ b/buffer/bs_capacitycheck.c
@@ -10,3 +10,31 @@ int bs_capacitycheck(struct bytestream* bs,size_t capacity) {
   if (bs->max - bs->cur < capacity) return 0;	// not EOF but less than that many bytes left
   return 1;
 }
+
+#ifdef UNITTEST
+#include <assert.h>
+
+#undef UNITTEST
+#include "buffer/bs_err.c"
+
+int main() {
+  struct bytestream bs;
+  // test basic functionality
+  bs.cur=0; bs.max=100;
+  assert(bs_capacitycheck(&bs, 100) == 1);
+  assert(bs_err(&bs) == 0);
+
+  bs.cur=50; bs.max=100;
+  assert(bs_capacitycheck(&bs, 50) == 1);
+  assert(bs_err(&bs) == 0);
+
+  assert(bs_capacitycheck(&bs, 51) == 0);
+  assert(bs_err(&bs) == 0);	// capacitycheck should not set error. capacityassert should
+
+  // try to provoke a numeric overflow
+  bs.cur=100; bs.max=(size_t)-1;
+  assert(bs_capacitycheck(&bs, (size_t)-50) == 0);
+  assert(bs_err(&bs) == 0);	// should still not set an error
+}
+
+#endif