diff --git a/CMakeLists.txt b/CMakeLists.txt
index a688f1649..f77a46033 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -265,6 +265,12 @@ set (wsjtx_CXXSRCS
   WSPRBandHopping.cpp
   )
 
+set (wsjt_CXXSRCS
+  lib/crc12.cpp
+  )
+# deal with a GCC v6 UB error message
+set_source_files_properties (lib/crc12.cpp PROPERTIES COMPILE_FLAGS -fpermissive)
+
 if (WIN32)
   set (wsjt_CXXSRCS
     ${wsjt_CXXSRCS}
@@ -1081,7 +1087,7 @@ target_link_libraries (ldpcsim40 wsjt_fort wsjt_cxx)
 add_executable (ldpcsim144 lib/ldpcsim144.f90 wsjtx.rc)
 target_link_libraries (ldpcsim144 wsjt_fort wsjt_cxx)
 
-add_executable (ldpcsim168 lib/fsk4hf/ldpcsim168.f90 lib/crc12.cpp wsjtx.rc)
+add_executable (ldpcsim168 lib/fsk4hf/ldpcsim168.f90 wsjtx.rc)
 target_link_libraries (ldpcsim168 wsjt_fort wsjt_cxx)
 
 add_executable (msk144sim lib/msk144sim.f90 wsjtx.rc)