diff --git a/src/variant.cpp b/src/variant.cpp
index 7b73771..542ee66 100644
--- a/src/variant.cpp
+++ b/src/variant.cpp
@@ -431,7 +431,14 @@ bool  variant::as_bool()const
    switch( get_type() )
    {
       case string_type:
-          return **reinterpret_cast<const const_string_ptr*>(this) == "true"; 
+      {
+          const string& s = **reinterpret_cast<const const_string_ptr*>(this);
+          if( s == "true" )
+             return true;
+          if( s == "false" )
+             return false;
+          FC_THROW_EXCEPTION( bad_cast_exception, "Cannot convert string to bool (only \"true\" or \"false\" can be converted)" );
+      }
       case double_type:
           return *reinterpret_cast<const double*>(this) != 0.0;
       case int64_type: