diff --git a/src/Rules/SinceVersionRule.php b/src/Rules/SinceVersionRule.php
index 80d21b5..a83f437 100644
--- a/src/Rules/SinceVersionRule.php
+++ b/src/Rules/SinceVersionRule.php
@@ -229,7 +229,60 @@ private static function getRequiresAtLeastValue( ?string $pluginFile ): string {
 			return self::getRequiresAtLeastHeader( $path );
 		}
 
-		throw new \RuntimeException( 'No plugin or theme file found' );
+		if ( is_string( $pluginFile ) ) {
+			throw new \RuntimeException(
+				sprintf(
+					'WPCompat: The plugin file "%s" set in the WPCompat.pluginFile parameter was not found.',
+					$pluginFile
+				)
+			);
+		}
+
+		// The directory name doesn't always match the main plugin file name, for example
+		// in a git worktree, so fall back to the same discovery that WordPress uses.
+		if ( is_string( $cwd ) ) {
+			$path = self::findPluginFile( $cwd );
+
+			if ( is_string( $path ) ) {
+				return self::getRequiresAtLeastHeader( $path );
+			}
+		}
+
+		throw new \RuntimeException(
+			sprintf(
+				'WPCompat: Could not find a plugin or theme file in %s. Set the WPCompat.pluginFile or WPCompat.requiresAtLeast parameter in your PHPStan config file.',
+				is_string( $cwd ) ? $cwd : 'the current directory'
+			)
+		);
+	}
+
+	/**
+	 * The logic in this method matches the logic in WordPress core's get_plugins() function.
+	 */
+	private static function findPluginFile( string $dir ): ?string {
+		$files = glob( "{$dir}/*.php" );
+
+		if ( ! is_array( $files ) ) {
+			return null;
+		}
+
+		sort( $files );
+
+		foreach ( $files as $file ) {
+			$contents = file_get_contents( $file, false, null, 0, 8 * 1024 );
+
+			if ( ! is_string( $contents ) ) {
+				continue;
+			}
+
+			$contents = str_replace( "\r", "\n", $contents );
+
+			if ( preg_match( '/^[ \t\/*#@]*Plugin Name:(.*)$/mi', $contents, $match ) === 1 && trim( $match[1] ) !== '' ) {
+				return $file;
+			}
+		}
+
+		return null;
 	}
 
 	/**
diff --git a/tests/Unit/HeaderTest.php b/tests/Unit/HeaderTest.php
index 699d019..728527d 100644
--- a/tests/Unit/HeaderTest.php
+++ b/tests/Unit/HeaderTest.php
@@ -57,4 +57,43 @@ public function dataMinimumVersions(): array {
 			],
 		];
 	}
+
+	public function testPluginFileIsFoundWhenDirectoryNameDoesNotMatch(): void {
+		$since = $this->createRuleInDirectory( __DIR__ . '/data/worktree' );
+
+		self::assertSame( '5.5.0', $since->getMinVersion() );
+	}
+
+	public function testMissingPluginFileThrowsHelpfulException(): void {
+		$this->expectException( \RuntimeException::class );
+		$this->expectExceptionMessage( 'Set the WPCompat.pluginFile or WPCompat.requiresAtLeast parameter' );
+
+		$this->createRuleInDirectory( __DIR__ . '/data/no-plugin' );
+	}
+
+	public function testMissingConfiguredPluginFileThrowsHelpfulException(): void {
+		$this->expectException( \RuntimeException::class );
+		$this->expectExceptionMessage( 'The plugin file "does-not-exist.php" set in the WPCompat.pluginFile parameter was not found' );
+
+		new SinceVersionRule(
+			null,
+			'does-not-exist.php',
+			self::$reflectionProvider
+		);
+	}
+
+	private function createRuleInDirectory( string $dir ): SinceVersionRule {
+		$cwd = (string) getcwd();
+		chdir( $dir );
+
+		try {
+			return new SinceVersionRule(
+				null,
+				null,
+				self::$reflectionProvider
+			);
+		} finally {
+			chdir( $cwd );
+		}
+	}
 }
diff --git a/tests/Unit/data/no-plugin/functions.php b/tests/Unit/data/no-plugin/functions.php
new file mode 100644
index 0000000..61f0f4b
--- /dev/null
+++ b/tests/Unit/data/no-plugin/functions.php
@@ -0,0 +1,4 @@
+<?php
+/**
+ * Not a plugin.
+ */
diff --git a/tests/Unit/data/worktree/functions.php b/tests/Unit/data/worktree/functions.php
new file mode 100644
index 0000000..b2c4ebb
--- /dev/null
+++ b/tests/Unit/data/worktree/functions.php
@@ -0,0 +1,4 @@
+<?php
+/**
+ * Not the main plugin file.
+ */
diff --git a/tests/Unit/data/worktree/my-plugin.php b/tests/Unit/data/worktree/my-plugin.php
new file mode 100644
index 0000000..050b114
--- /dev/null
+++ b/tests/Unit/data/worktree/my-plugin.php
@@ -0,0 +1,5 @@
+<?php
+/**
+ * Plugin Name: My Plugin
+ * Requires at least: 5.5
+ */