diff --git a/cmake/compiler_warnings.cmake b/cmake/compiler_warnings.cmake
index 7a68711bc6..ebabc71dba 100644
--- a/cmake/compiler_warnings.cmake
+++ b/cmake/compiler_warnings.cmake
@@ -27,6 +27,7 @@ set(CLANG_ONLY_WARNINGS
   "-Wcast-qual"
   "-Wimplicit-fallthrough"
   "-Wextra-semi"
+  "-Wignored-qualifiers"
 )
 
 set(MSVC_WARNINGS "/W3")
diff --git a/src/math/lp/dioph_eq.cpp b/src/math/lp/dioph_eq.cpp
index ba38d2227f..68aafaab6c 100644
--- a/src/math/lp/dioph_eq.cpp
+++ b/src/math/lp/dioph_eq.cpp
@@ -1515,7 +1515,7 @@ namespace lp {
             return ret;
         }
 
-        const unsigned sub_index(unsigned k) const {
+        unsigned sub_index(unsigned k) const {
             return m_k2s[k];
         }