diff --git a/src/cli.cc b/src/cli.cc
index 54cea4a..80964fd 100644
--- a/src/cli.cc
+++ b/src/cli.cc
@@ -146,6 +146,8 @@ void ot::print_comments(const std::list<std::string>& comments, FILE* output)
 	std::string local;
 	bool info_lost = false;
 	bool bad_comments = false;
+	bool has_newline = false;
+	bool has_control = false;
 	for (const std::string& comment : comments) {
 		ot::status rc = from_utf8(comment, local);
 		if (rc == ot::st::information_lost) {
@@ -154,6 +156,12 @@ void ot::print_comments(const std::list<std::string>& comments, FILE* output)
 			bad_comments = true;
 			continue;
 		}
+		for (unsigned char c : comment) {
+			if (c == '\n')
+				has_newline = true;
+			else if (c < 0x20)
+				has_control = true;
+		}
 		fwrite(local.data(), 1, local.size(), output);
 		putchar('\n');
 	}
@@ -161,6 +169,11 @@ void ot::print_comments(const std::list<std::string>& comments, FILE* output)
 		fputs("warning: Some tags have been transliterated to your system encoding.\n", stderr);
 	if (bad_comments)
 		fputs("warning: Some tags are not properly encoded and have not been displayed.\n", stderr);
+	if (has_newline)
+		fputs("warning: Some tags contain newline characters. "
+		      "These are not supported by --set-all.\n", stderr);
+	if (has_control)
+		fputs("warning: Some tags contain control characters.\n", stderr);
 }
 
 /**
diff --git a/t/opustags.t b/t/opustags.t
index d43814c..563575b 100755
--- a/t/opustags.t
+++ b/t/opustags.t
@@ -8,7 +8,7 @@ use strict;
 use warnings;
 use utf8;
 
-use Test::More tests => 33;
+use Test::More tests => 34;
 
 use Digest::MD5;
 use File::Basename;
@@ -143,6 +143,14 @@ error: Invalid comment 'FOO'.
 EOF
 is(md5('out.opus'), '66780307a6081523dc9040f3c47b0448', 'the file did not change');
 
+is_deeply(opustags('out.opus', '-D', '-a', "X=foo\nbar\tquux"), [<<'END_OUT', <<'END_ERR', 0], 'control characters');
+X=foo
+bar	quux
+END_OUT
+warning: Some tags contain newline characters. These are not supported by --set-all.
+warning: Some tags contain control characters.
+END_ERR
+
 is_deeply(opustags(qw(-i out.opus -s fatal=yes -s FOO -s BAR)), ['', <<'EOF', 256], 'bad tag with --set');
 error: Invalid comment 'FOO'.
 EOF