diff --git a/src/util/bit_vector.cpp b/src/util/bit_vector.cpp
index 210d230bc..2328a5849 100644
--- a/src/util/bit_vector.cpp
+++ b/src/util/bit_vector.cpp
@@ -116,7 +116,7 @@ void bit_vector::shift_right(unsigned k) {
     }
 }
 
-bool bit_vector::operator==(bit_vector const & source) {
+bool bit_vector::operator==(bit_vector const & source) const {
     if (m_num_bits != source.m_num_bits)
         return false;
     unsigned n = num_words();
@@ -129,6 +129,7 @@ bool bit_vector::operator==(bit_vector const & source) {
     }
     unsigned bit_rest = source.m_num_bits % 32;
     unsigned mask = (1 << bit_rest) - 1;
+    if (mask == 0) mask = UINT_MAX;
     return (m_data[i] & mask) == (source.m_data[i] & mask);
 }
 
diff --git a/src/util/bit_vector.h b/src/util/bit_vector.h
index 9560af7e2..2e7becee7 100644
--- a/src/util/bit_vector.h
+++ b/src/util/bit_vector.h
@@ -171,9 +171,9 @@ public:
             resize(sz, val);
     }
 
-    bool operator==(bit_vector const & other);
+    bool operator==(bit_vector const & other) const;
 
-    bool operator!=(bit_vector const & other) { return !operator==(other); }
+    bool operator!=(bit_vector const & other) const { return !operator==(other); }
 
     bit_vector & operator=(bit_vector const & source) {
         m_num_bits = source.m_num_bits;