diff --git a/psycopg/typecast_datetime.c b/psycopg/typecast_datetime.c
index 7c1cfa45..18e9d0d8 100644
--- a/psycopg/typecast_datetime.c
+++ b/psycopg/typecast_datetime.c
@@ -239,7 +239,7 @@ typecast_PYINTERVAL_cast(const char *str, Py_ssize_t len, PyObject *curs)
 
         case '0': case '1': case '2': case '3': case '4':
         case '5': case '6': case '7': case '8': case '9':
-            v = v*10 + (double)*str - (double)'0';
+            v = v * 10.0 + (double)(*str - '0');
             if (part == 6){
                 denominator *= 10;
             }
diff --git a/psycopg/typecast_mxdatetime.c b/psycopg/typecast_mxdatetime.c
index e61224df..bf69af5a 100644
--- a/psycopg/typecast_mxdatetime.c
+++ b/psycopg/typecast_mxdatetime.c
@@ -142,7 +142,7 @@ typecast_MXINTERVAL_cast(const char *str, Py_ssize_t len, PyObject *curs)
 
         case '0': case '1': case '2': case '3': case '4':
         case '5': case '6': case '7': case '8': case '9':
-            v = v*10 + (double)*str - (double)'0';
+            v = v * 10.0 + (double)(*str - '0');
             Dprintf("typecast_MXINTERVAL_cast: v = %f", v);
             if (part == 6){
                 denominator *= 10;