diff --git a/be/src/exprs/aggregate-functions.cc b/be/src/exprs/aggregate-functions.cc
index 05f9c561d..e514d902c 100644
--- a/be/src/exprs/aggregate-functions.cc
+++ b/be/src/exprs/aggregate-functions.cc
@@ -127,14 +127,14 @@ namespace impala {
 // 'dst' will be set to a null string. This allows execution to continue until the
 // next time GetQueryStatus() is called (see IMPALA-2756).
 static void AllocBuffer(FunctionContext* ctx, StringVal* dst, size_t buf_len) {
-  DCHECK_GT(buf_len, 0);
   uint8_t* ptr = ctx->Allocate(buf_len);
-  if (UNLIKELY(ptr == NULL)) {
+  if (UNLIKELY(ptr == NULL && buf_len != 0)) {
     DCHECK(!ctx->impl()->state()->GetQueryStatus().ok());
     *dst = StringVal::null();
   } else {
     *dst = StringVal(ptr, buf_len);
-    memset(ptr, 0, buf_len);
+    // Avoid memset() with NULL ptr as it's undefined.
+    if (LIKELY(ptr != NULL)) memset(ptr, 0, buf_len);
   }
 }
 
@@ -142,14 +142,14 @@ static void AllocBuffer(FunctionContext* ctx, StringVal* dst, size_t buf_len) {
 // 'buf_len' bytes and copies the content of StringVal 'src' into it.
 // If allocation fails, 'dst' will be set to a null string.
 static void CopyStringVal(FunctionContext* ctx, const StringVal& src, StringVal* dst) {
-  DCHECK_GT(src.len, 0);
   uint8_t* copy = ctx->Allocate(src.len);
-  if (UNLIKELY(copy == NULL)) {
+  if (UNLIKELY(copy == NULL && src.len != 0)) {
     DCHECK(!ctx->impl()->state()->GetQueryStatus().ok());
     *dst = StringVal::null();
   } else {
     *dst = StringVal(copy, src.len);
-    memcpy(dst->ptr, src.ptr, src.len);
+    // Avoid memcpy() to NULL ptr as it's undefined.
+    if (LIKELY(dst->ptr != NULL)) memcpy(dst->ptr, src.ptr, src.len);
   }
 }
 
diff --git a/testdata/workloads/functional-query/queries/QueryTest/aggregation.test b/testdata/workloads/functional-query/queries/QueryTest/aggregation.test
index f8a3fbea1..5389b04f3 100644
--- a/testdata/workloads/functional-query/queries/QueryTest/aggregation.test
+++ b/testdata/workloads/functional-query/queries/QueryTest/aggregation.test
@@ -154,6 +154,15 @@ from alltypesagg where day is not null
 bigint, string, string, string, string
 ====
 ---- QUERY
+# Test for IMPALA-3018. Verify update() function of min() handles
+# zero-length string correctly.
+select min(str) from (values ('aaa' as str), ('')) as tmp
+---- RESULTS
+''
+---- TYPES
+String
+====
+---- QUERY
 # grouping by different data types, with NULLs
 select tinyint_col, count(*) from alltypesagg where day is not null group by 1 order by 1
 ---- RESULTS