diff --git a/src/backend/commands/copy.c b/src/backend/commands/copy.c
index 6b5bcd83c5..95fec8d386 100644
--- a/src/backend/commands/copy.c
+++ b/src/backend/commands/copy.c
@@ -3243,7 +3243,17 @@ CopyReadAttributesText(CopyState cstate)
 		start_ptr = cur_ptr;
 		cstate->raw_fields[fieldno] = output_ptr;
 
-		/* Scan data for field */
+		/*
+		 * Scan data for field.
+		 *
+		 * Note that in this loop, we are scanning to locate the end of field
+		 * and also speculatively performing de-escaping.  Once we find the
+		 * end-of-field, we can match the raw field contents against the null
+		 * marker string.  Only after that comparison fails do we know that
+		 * de-escaping is actually the right thing to do; therefore we *must
+		 * not* throw any syntax errors before we've done the null-marker
+		 * check.
+		 */
 		for (;;)
 		{
 			char		c;
@@ -3356,26 +3366,29 @@ CopyReadAttributesText(CopyState cstate)
 			*output_ptr++ = c;
 		}
 
-		/* Terminate attribute value in output area */
-		*output_ptr++ = '\0';
-
-		/*
-		 * If we de-escaped a non-7-bit-ASCII char, make sure we still have
-		 * valid data for the db encoding. Avoid calling strlen here for the
-		 * sake of efficiency.
-		 */
-		if (saw_non_ascii)
-		{
-			char	   *fld = cstate->raw_fields[fieldno];
-
-			pg_verifymbstr(fld, output_ptr - (fld + 1), false);
-		}
-
 		/* Check whether raw input matched null marker */
 		input_len = end_ptr - start_ptr;
 		if (input_len == cstate->null_print_len &&
 			strncmp(start_ptr, cstate->null_print, input_len) == 0)
 			cstate->raw_fields[fieldno] = NULL;
+		else
+		{
+			/*
+			 * At this point we know the field is supposed to contain data.
+			 *
+			 * If we de-escaped any non-7-bit-ASCII chars, make sure the
+			 * resulting string is valid data for the db encoding.
+			 */
+			if (saw_non_ascii)
+			{
+				char	   *fld = cstate->raw_fields[fieldno];
+
+				pg_verifymbstr(fld, output_ptr - fld, false);
+			}
+		}
+
+		/* Terminate attribute value in output area */
+		*output_ptr++ = '\0';
 
 		fieldno++;
 		/* Done if we hit EOL instead of a delim */
diff --git a/src/test/regress/expected/copy2.out b/src/test/regress/expected/copy2.out
index 8e2bc0c250..2449eefb16 100644
--- a/src/test/regress/expected/copy2.out
+++ b/src/test/regress/expected/copy2.out
@@ -239,6 +239,22 @@ a\.
 \.b
 c\.d
 "\."
+-- test handling of nonstandard null marker that violates escaping rules
+CREATE TEMP TABLE testnull(a int, b text);
+INSERT INTO testnull VALUES (1, E'\\0'), (NULL, NULL);
+COPY testnull TO stdout WITH NULL AS E'\\0';
+1	\\0
+\0	\0
+COPY testnull FROM stdin WITH NULL AS E'\\0';
+SELECT * FROM testnull;
+ a  | b  
+----+----
+  1 | \0
+    | 
+ 42 | \0
+    | 
+(4 rows)
+
 DROP TABLE x, y;
 DROP FUNCTION fn_x_before();
 DROP FUNCTION fn_x_after();
diff --git a/src/test/regress/sql/copy2.sql b/src/test/regress/sql/copy2.sql
index 6322c8fba4..1961446fdb 100644
--- a/src/test/regress/sql/copy2.sql
+++ b/src/test/regress/sql/copy2.sql
@@ -164,6 +164,21 @@ c\.d
 
 COPY testeoc TO stdout CSV;
 
+-- test handling of nonstandard null marker that violates escaping rules
+
+CREATE TEMP TABLE testnull(a int, b text);
+INSERT INTO testnull VALUES (1, E'\\0'), (NULL, NULL);
+
+COPY testnull TO stdout WITH NULL AS E'\\0';
+
+COPY testnull FROM stdin WITH NULL AS E'\\0';
+42	\\0
+\0	\0
+\.
+
+SELECT * FROM testnull;
+
+
 DROP TABLE x, y;
 DROP FUNCTION fn_x_before();
 DROP FUNCTION fn_x_after();