diff --git a/src/main/java/moe/yushi/authlibinjector/AuthlibInjector.java b/src/main/java/moe/yushi/authlibinjector/AuthlibInjector.java
index 972e741..456ec65 100644
--- a/src/main/java/moe/yushi/authlibinjector/AuthlibInjector.java
+++ b/src/main/java/moe/yushi/authlibinjector/AuthlibInjector.java
@@ -99,7 +99,7 @@ public final class AuthlibInjector {
 		String apiRoot = System.getProperty(PROP_API_ROOT);
 		if (apiRoot == null) return empty();
 
-		apiRoot = appendSuffixSlash(apiRoot);
+		apiRoot = parseInputUrl(apiRoot);
 		Logging.CONFIG.info("API root: " + apiRoot);
 		warnIfHttp(apiRoot);
 
@@ -155,6 +155,16 @@ public final class AuthlibInjector {
 		}
 	}
 
+	private static String parseInputUrl(String url) {
+		String lowercased = url.toLowerCase();
+		if (!lowercased.startsWith("http://") && !lowercased.startsWith("https://")) {
+			url = "https://" + url;
+		}
+
+		url = appendSuffixSlash(url);
+		return url;
+	}
+
 	private static ClassTransformer createTransformer(YggdrasilConfiguration config) {
 		ClassTransformer transformer = new ClassTransformer();
 		transformer.debugSaveClass = "true".equals(System.getProperty(PROP_DUMP_CLASS));