diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index d750290f13..bb4d985f35 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -3597,9 +3597,11 @@ include_dir 'conf.d'
        </para>
        <para>
         This parameter can only be set in the <filename>postgresql.conf</filename>
-        file or on the server command line.  It is ignored unless
+        file or on the server command line.  It is only used if
         <varname>archive_mode</varname> was enabled at server start and
-        <varname>archive_library</varname> is set to an empty string.
+        <varname>archive_library</varname> is set to an empty string.  If both
+        <varname>archive_command</varname> and <varname>archive_library</varname>
+        are set, archiving will fail.
         If <varname>archive_command</varname> is an empty string (the default) while
         <varname>archive_mode</varname> is enabled (and <varname>archive_library</varname>
         is set to an empty string), WAL archiving is temporarily
@@ -3624,7 +3626,9 @@ include_dir 'conf.d'
        <para>
         The library to use for archiving completed WAL file segments.  If set to
         an empty string (the default), archiving via shell is enabled, and
-        <xref linkend="guc-archive-command"/> is used.  Otherwise, the specified
+        <xref linkend="guc-archive-command"/> is used.  If both
+        <varname>archive_command</varname> and <varname>archive_library</varname>
+        are set, archiving will fail.  Otherwise, the specified
         shared library is used for archiving.  For more information, see
         <xref linkend="backup-archiving-wal"/> and
         <xref linkend="archive-modules"/>.
diff --git a/src/backend/postmaster/pgarch.c b/src/backend/postmaster/pgarch.c
index 3868cd7bd3..56dcc0dce5 100644
--- a/src/backend/postmaster/pgarch.c
+++ b/src/backend/postmaster/pgarch.c
@@ -838,6 +838,12 @@ LoadArchiveLibrary(void)
 
 	memset(&ArchiveContext, 0, sizeof(ArchiveModuleCallbacks));
 
+	if (XLogArchiveLibrary[0] != '\0' && XLogArchiveCommand[0] != '\0')
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("both archive_command and archive_library specified"),
+				 errdetail("Only one of archive_command, archive_library may be set.")));
+
 	/*
 	 * If shell archiving is enabled, use our special initialization function.
 	 * Otherwise, load the library and call its _PG_archive_module_init().