Assembler is a critical component of the compiler toolchain, which has been less tested than the other components. Unfortunately, current grammar-based fuzzing techniques suffer from several challenges when testing assemblers. First, each different assembler accepts different grammar rules and syntaxes, and there are no existing assembly grammar specifications. Second, not every assembler is open-source, which makes it difficult to extract grammar rules from the source code. While existing black-box grammar inference approaches are applicable to such closed-source assemblers, they suffer from the scalability issue, which renders them impractical for testing assemblers. To address these challenges, we propose a novel way to test assemblers by automatically inferring their grammar rules with only a few queries to the target assemblers by leveraging their error messages. The key insight is that assembly error messages often deliver useful information to infer the underlying grammar rules. We have implemented our technique in a tool named AsFuzzer, and evaluated it on 4 real-world assemblers including Clang-integrated assembler (Clang), GNU assembler (GAS), Intel’s assembler (ICC), and Microsoft macro assembler (MASM). With AsFuzzer, we have successfully found 497 buggy instruction opcodes for six popular architectures, and reported them to the developers.