diff --git a/src/box/errcode.h b/src/box/errcode.h
index 3065b194889d..6f6e28c6c121 100644
--- a/src/box/errcode.h
+++ b/src/box/errcode.h
@@ -80,7 +80,7 @@ struct errcode_record {
 	/* 25 */_(ER_UPDATE_SPLICE,		"SPLICE error on field %s: %s") \
 	/* 26 */_(ER_UPDATE_ARG_TYPE,		"Argument type in operation '%c' on field %s does not match field type: expected %s") \
 	/* 27 */_(ER_FORMAT_MISMATCH_INDEX_PART, "Field %s has type '%s' in space format, but type '%s' in index definition") \
-	/* 28 */_(ER_UNKNOWN_UPDATE_OP,		"Unknown UPDATE operation") \
+	/* 28 */_(ER_UNKNOWN_UPDATE_OP,		"Unknown UPDATE operation #%d: %s") \
 	/* 29 */_(ER_UPDATE_FIELD,		"Field %s UPDATE error: %s") \
 	/* 30 */_(ER_FUNCTION_TX_ACTIVE,	"Transaction is active at return from function") \
 	/* 31 */_(ER_KEY_PART_COUNT,		"Invalid key part count (expected [0..%u], got %u)") \
diff --git a/src/box/xrow_update.c b/src/box/xrow_update.c
index 0cee50017ae4..e7a98073a397 100644
--- a/src/box/xrow_update.c
+++ b/src/box/xrow_update.c
@@ -177,8 +177,8 @@ xrow_update_read_ops(struct xrow_update *update, const char *expr,
 	}
 	struct xrow_update_op *op = update->ops;
 	struct xrow_update_op *ops_end = op + update->op_count;
-	for (; op < ops_end; op++) {
-		if (xrow_update_op_decode(op, update->index_base, dict,
+	for (int i = 1; op < ops_end; op++, i++) {
+		if (xrow_update_op_decode(op, i, update->index_base, dict,
 					  &expr) != 0)
 			return -1;
 		/*
diff --git a/src/box/xrow_update_field.c b/src/box/xrow_update_field.c
index 7debc1930345..deee91738dcd 100644
--- a/src/box/xrow_update_field.c
+++ b/src/box/xrow_update_field.c
@@ -595,7 +595,7 @@ static const struct xrow_update_op_meta op_delete = {
 };
 
 static inline const struct xrow_update_op_meta *
-xrow_update_op_by(char opcode)
+xrow_update_op_by(char opcode, int op_num)
 {
 	switch (opcode) {
 	case '=':
@@ -614,9 +614,12 @@ xrow_update_op_by(char opcode)
 	case '!':
 		return &op_insert;
 	default:
-		diag_set(ClientError, ER_UNKNOWN_UPDATE_OP);
-		return NULL;
+		goto error;
 	}
+error:
+	diag_set(ClientError, ER_UNKNOWN_UPDATE_OP, op_num,
+		 tt_sprintf("\"%c\"", opcode));
+	return NULL;
 }
 
 int
@@ -637,7 +640,7 @@ xrow_update_op_next_token(struct xrow_update_op *op)
 }
 
 int
-xrow_update_op_decode(struct xrow_update_op *op, int index_base,
+xrow_update_op_decode(struct xrow_update_op *op, int op_num, int index_base,
 		      struct tuple_dictionary *dict, const char **expr)
 {
 	if (mp_typeof(**expr) != MP_ARRAY) {
@@ -657,11 +660,14 @@ xrow_update_op_decode(struct xrow_update_op *op, int index_base,
 		return -1;
 	}
 	op->opcode = *mp_decode_str(expr, &len);
-	op->meta = xrow_update_op_by(op->opcode);
+	op->meta = xrow_update_op_by(op->opcode, op_num);
 	if (op->meta == NULL)
 		return -1;
 	if (arg_count != op->meta->arg_count) {
-		diag_set(ClientError, ER_UNKNOWN_UPDATE_OP);
+		const char *str = tt_sprintf("wrong number of arguments, "\
+					     "expected %u, got %u",
+					     op->meta->arg_count, arg_count);
+		diag_set(ClientError, ER_UNKNOWN_UPDATE_OP, op_num, str);
 		return -1;
 	}
 	/*
diff --git a/src/box/xrow_update_field.h b/src/box/xrow_update_field.h
index 387c265662ed..451a23024f07 100644
--- a/src/box/xrow_update_field.h
+++ b/src/box/xrow_update_field.h
@@ -223,6 +223,7 @@ xrow_update_op_next_token(struct xrow_update_op *op);
 /**
  * Decode an update operation from MessagePack.
  * @param[out] op Update operation.
+ * @param op_num Ordinal number of the operation.
  * @param index_base Field numbers base: 0 or 1.
  * @param dict Dictionary to lookup field number by a name.
  * @param expr MessagePack.
@@ -231,7 +232,7 @@ xrow_update_op_next_token(struct xrow_update_op *op);
  * @retval -1 Client error.
  */
 int
-xrow_update_op_decode(struct xrow_update_op *op, int index_base,
+xrow_update_op_decode(struct xrow_update_op *op, int op_num, int index_base,
 		      struct tuple_dictionary *dict, const char **expr);
 
 /**
diff --git a/test/box/gh-4511-access-settings-from-any-frontend.result b/test/box/gh-4511-access-settings-from-any-frontend.result
index 64532a1baeff..ed340d053f56 100644
--- a/test/box/gh-4511-access-settings-from-any-frontend.result
+++ b/test/box/gh-4511-access-settings-from-any-frontend.result
@@ -196,15 +196,18 @@ s:update('sql_defer_foreign_keys', {{}})
  | ...
 s:update('sql_defer_foreign_keys', {{'='}})
  | ---
- | - error: Unknown UPDATE operation
+ | - error: 'Unknown UPDATE operation #1: wrong number of arguments, expected 3, got
+ |     1'
  | ...
 s:update('sql_defer_foreign_keys', {{'=', 'value'}})
  | ---
- | - error: Unknown UPDATE operation
+ | - error: 'Unknown UPDATE operation #1: wrong number of arguments, expected 3, got
+ |     2'
  | ...
 s:update('sql_defer_foreign_keys', {{'=', 'value', true, 1}})
  | ---
- | - error: Unknown UPDATE operation
+ | - error: 'Unknown UPDATE operation #1: wrong number of arguments, expected 3, got
+ |     4'
  | ...
 
 s:update('sql_defer_foreign_keys', {{'+', 'value', 2}})
diff --git a/test/box/update.result b/test/box/update.result
index d1dc06eae0d4..7fc0b02c68ad 100644
--- a/test/box/update.result
+++ b/test/box/update.result
@@ -826,11 +826,13 @@ s:update({0}, {{}})
 ...
 s:update({0}, {{'+'}})
 ---
-- error: Unknown UPDATE operation
+- error: 'Unknown UPDATE operation #1: wrong number of arguments, expected 3, got
+    1'
 ...
 s:update({0}, {{'+', 0}})
 ---
-- error: Unknown UPDATE operation
+- error: 'Unknown UPDATE operation #1: wrong number of arguments, expected 3, got
+    2'
 ...
 s:update({0}, {{'+', '+', '+'}})
 ---
diff --git a/test/engine/upsert.result b/test/engine/upsert.result
index 47da307fa394..5610c675d94d 100644
--- a/test/engine/upsert.result
+++ b/test/engine/upsert.result
@@ -1809,7 +1809,7 @@ space:upsert({1}, {{'!', 2, 100}}) -- must fail on checking tuple
 ...
 space:upsert({'a'}, {{'a', 2, 100}}) -- must fail on checking ops
 ---
-- error: Unknown UPDATE operation
+- error: 'Unknown UPDATE operation #1: "a"'
 ...
 space:upsert({'a'}, {{'!', 2, 'ups1'}}) -- 'fast' upsert via insert in one index
 ---
@@ -2117,19 +2117,22 @@ test(t, {{'+', 3, 3}, {'&', 3, 'a'}}, {{1, '1', 1, '1'}})
 ...
 test(t, {{'+', 3}, {'&', 3, 'a'}}, {{1, '1', 1, '1'}})
 ---
-- error: Unknown UPDATE operation
+- error: 'Unknown UPDATE operation #1: wrong number of arguments, expected 3, got
+    2'
 ...
 test(t, {{':', 3, 3}}, {{1, '1', 1, '1'}})
 ---
-- error: Unknown UPDATE operation
+- error: 'Unknown UPDATE operation #1: wrong number of arguments, expected 5, got
+    3'
 ...
 test(t, {{':', 3, 3, 3}}, {{1, '1', 1, '1'}})
 ---
-- error: Unknown UPDATE operation
+- error: 'Unknown UPDATE operation #1: wrong number of arguments, expected 5, got
+    4'
 ...
 test(t, {{'?', 3, 3}}, {{1, '1', 1, '1'}})
 ---
-- error: Unknown UPDATE operation
+- error: 'Unknown UPDATE operation #1: "?"'
 ...
 'dump ' .. anything_to_string(box.space.s:select{})
 ---
diff --git a/test/vinyl/gh.result b/test/vinyl/gh.result
index 78cb2a28d9f8..59578ebddb98 100644
--- a/test/vinyl/gh.result
+++ b/test/vinyl/gh.result
@@ -369,7 +369,8 @@ s:select()
 ...
 s:upsert({1, 'test', 'failed'}, {{'=', 3, 33}, {'=', 4, nil}})
 ---
-- error: Unknown UPDATE operation
+- error: 'Unknown UPDATE operation #2: wrong number of arguments, expected 3, got
+    2'
 ...
 s:select()
 ---