diff --git a/src/grib_expression_class_accessor.c b/src/grib_expression_class_accessor.c
index 07c9159a7..372888cc8 100644
--- a/src/grib_expression_class_accessor.c
+++ b/src/grib_expression_class_accessor.c
@@ -125,13 +125,16 @@ static string evaluate_string(grib_expression* g, grib_handle* h, char* buf, siz
         start += *size;
 
     if (e->length != 0) {
-        memcpy(buf, mybuf + start, e->length);
+        if (start > 0)
+            memcpy(buf, mybuf + start, e->length);
         buf[e->length] = 0;
     }
     else {
-        memcpy(buf, mybuf, *size);
-        if (*size == 1024)
-            *size = *size - 1; /* ECC-336 */
+        if (*size > 0) {
+            memcpy(buf, mybuf, *size);
+            if (*size == 1024)
+                *size = *size - 1; /* ECC-336 */
+        }
         buf[*size] = 0;
     }
     return buf;
diff --git a/src/grib_expression_class_string_compare.c b/src/grib_expression_class_string_compare.c
index cdae91f55..eb3501ad9 100644
--- a/src/grib_expression_class_string_compare.c
+++ b/src/grib_expression_class_string_compare.c
@@ -107,8 +107,8 @@ static int evaluate_long(grib_expression* g, grib_handle* h, long* lres)
     size_t l1 = sizeof(b1);
     char b2[1024];
     size_t l2 = sizeof(b2);
-    const char* v1;
-    const char* v2;
+    const char* v1 = NULL;
+    const char* v2 = NULL;
 
     grib_expression_string_compare* e = (grib_expression_string_compare*)g;