Invalid icon for server - Illegal base64 character a

Two of the server icons in my server list fail to load (from server list file and after pinging the server), the game throws the following exceptions in console:

[14:44:21] [Client thread/ERROR]: Invalid icon for server TrueSMP (play.truesmp.net)
java.lang.IllegalArgumentException: Illegal base64 character a
	at java.util.Base64$Decoder.decode0(Unknown Source) ~[?:1.8.0_171]
	at java.util.Base64$Decoder.decode(Unknown Source) ~[?:1.8.0_171]
	at clz.g(SourceFile:212) [1.13.1.jar:?]
	at clz.a(SourceFile:144) [1.13.1.jar:?]
	at chf.a(SourceFile:35) [1.13.1.jar:?]
	at chk.a(SourceFile:391) [1.13.1.jar:?]
	at chk.a(SourceFile:194) [1.13.1.jar:?]
	at clw.a(SourceFile:310) [1.13.1.jar:?]
	at ctx.a(SourceFile:792) [1.13.1.jar:?]
	at cfs.c(SourceFile:850) [1.13.1.jar:?]
	at cfs.a(SourceFile:395) [1.13.1.jar:?]
	at net.minecraft.client.main.Main.main(SourceFile:144) [1.13.1.jar:?]
[14:44:21] [Client thread/ERROR]: Invalid icon for server NoCoords (nocoords.com)
java.lang.IllegalArgumentException: Illegal base64 character a
	at java.util.Base64$Decoder.decode0(Unknown Source) ~[?:1.8.0_171]
	at java.util.Base64$Decoder.decode(Unknown Source) ~[?:1.8.0_171]
	at clz.g(SourceFile:212) [1.13.1.jar:?]
	at clz.a(SourceFile:144) [1.13.1.jar:?]
	at chf.a(SourceFile:35) [1.13.1.jar:?]
	at chk.a(SourceFile:391) [1.13.1.jar:?]
	at chk.a(SourceFile:194) [1.13.1.jar:?]
	at clw.a(SourceFile:310) [1.13.1.jar:?]
	at ctx.a(SourceFile:792) [1.13.1.jar:?]
	at cfs.c(SourceFile:850) [1.13.1.jar:?]
	at cfs.a(SourceFile:395) [1.13.1.jar:?]
	at net.minecraft.client.main.Main.main(SourceFile:144) [1.13.1.jar:?]

Image shows 1.12 on the left and 1.13.1 on the right. The issue was not present in 1.13 (and I did not test snapshots).