diff --git a/src/helpers.js b/src/helpers.js
index 022f471..7358771 100644
--- a/src/helpers.js
+++ b/src/helpers.js
@@ -410,6 +410,16 @@ export function decorateResult(
 		result[1] = symbols[result[1]];
 	}
 
+	// Capture the numeric value before formatting; a comma decimal separator
+	// (via separator or a locale such as de-DE) would otherwise make parseFloat
+	// read "1,5" as 1 and select the singular unit name.
+	let numericValue;
+	if (typeof result[0] === "string") {
+		numericValue = parseFloat(result[0]);
+	} else {
+		numericValue = result[0];
+	}
+
 	result[0] = applyNumberFormatting(
 		result[0],
 		locale,
@@ -427,15 +437,9 @@ export function decorateResult(
 		} else {
 			unit = BYTE;
 		}
-		let val;
-		if (typeof result[0] === "string") {
-			val = parseFloat(result[0]);
-		} else {
-			val = result[0];
-		}
 		// Determine singular/plural suffix
 		let suffix;
-		if (val === 1) {
+		if (numericValue === 1) {
 			suffix = EMPTY;
 		} else {
 			suffix = S;
diff --git a/tests/unit/filesize.test.js b/tests/unit/filesize.test.js
index 1e4ccda..4cb51cf 100644
--- a/tests/unit/filesize.test.js
+++ b/tests/unit/filesize.test.js
@@ -283,6 +283,11 @@ describe("filesize", () => {
 			);
 			assert.strictEqual(filesize(1234567890, { precision: 2, fullform: true }), "1.2 gigabytes");
 		});
+
+		it("should keep the plural fullform when a comma decimal separator is used", () => {
+			assert.strictEqual(filesize(1500000, { fullform: true, separator: "," }), "1,5 megabytes");
+			assert.strictEqual(filesize(1500000, { fullform: true, locale: "de-DE" }), "1,5 megabytes");
+		});
 	});
 
 	describe("Base and exponent options", () => {