diff --git a/lib/SizeFormatHelpers.js b/lib/SizeFormatHelpers.js
index 85bbd2c02..c4677f60b 100644
--- a/lib/SizeFormatHelpers.js
+++ b/lib/SizeFormatHelpers.js
@@ -7,6 +7,10 @@
 const SizeFormatHelpers = exports;
 
 SizeFormatHelpers.formatSize = size => {
+	if (typeof size !== "number" || Number.isNaN(size) === true) {
+		return "unknown size";
+	}
+
 	if (size <= 0) {
 		return "0 bytes";
 	}
diff --git a/test/SizeFormatHelpers.unittest.js b/test/SizeFormatHelpers.unittest.js
index 7d3f8c842..d641ee342 100644
--- a/test/SizeFormatHelpers.unittest.js
+++ b/test/SizeFormatHelpers.unittest.js
@@ -43,5 +43,10 @@ describe("SizeFormatHelpers", () => {
 				"1.2 GiB"
 			);
 		});
+
+		it("should handle undefined/NaN", () => {
+			should(SizeFormatHelpers.formatSize(undefined)).be.eql("unknown size");
+			should(SizeFormatHelpers.formatSize(NaN)).be.eql("unknown size");
+		});
 	});
 });