diff --git a/src/Base64.cc b/src/Base64.cc
index 1cf3f1b924..b88c7dff60 100644
--- a/src/Base64.cc
+++ b/src/Base64.cc
@@ -184,17 +184,17 @@ int Base64Converter::Decode(int len, const char* data, int* pblen, char** pbuf)
 		if ( dlen >= len )
 			break;
 
-		if ( data[dlen] == '=' )
+		unsigned char c = (unsigned char)data[dlen];
+		if ( c == '=' )
 			++base64_padding;
 
-		int k = base64_table[(unsigned char)data[dlen]];
+		int k = base64_table[c];
 		if ( k >= 0 )
 			base64_group[base64_group_next++] = k;
 		else
 			{
 			if ( ++errored == 1 )
-				IllegalEncoding(
-					util::fmt("character %d ignored by Base64 decoding", (int)(data[dlen])));
+				IllegalEncoding(util::fmt("character %d ignored by Base64 decoding", (int)c));
 			}
 
 		++dlen;
diff --git a/testing/btest/Baseline/bifs.decode_base64_errors/out b/testing/btest/Baseline/bifs.decode_base64_errors/out
new file mode 100644
index 0000000000..6fd2476e2f
--- /dev/null
+++ b/testing/btest/Baseline/bifs.decode_base64_errors/out
@@ -0,0 +1,7 @@
+### BTest baseline data generated by btest-diff. Do not edit. Use "btest -U/-u" to update. Requires BTest >= 0.63.
+error: character 94 ignored by Base64 decoding
+error: error in decoding string ^#@!@##$!===
+error: character 237 ignored by Base64 decoding
+error: error in decoding string ноп===
+PASS
+PASS
diff --git a/testing/btest/bifs/decode_base64_errors.zeek b/testing/btest/bifs/decode_base64_errors.zeek
new file mode 100644
index 0000000000..0becccb09b
--- /dev/null
+++ b/testing/btest/bifs/decode_base64_errors.zeek
@@ -0,0 +1,11 @@
+# @TEST-EXEC: zeek -b %INPUT >out 2>&1
+# @TEST-EXEC: btest-diff out
+
+event zeek_init()
+	{
+	local r1 = decode_base64("^#@!@##$!===");
+	print |r1| > 0 ? "FAIL" : "PASS";
+
+	local r2 = decode_base64("\xed\xee\xef===");
+	print |r2| > 0 ? "FAIL" : "PASS";
+	}