```typescript
/** The KEY=VALUE pairs of a config file. */
export function parseConfig(text: string): Record<string, string> {
  const config: Record<string, string> = {};
  for (const line of text.split("\n")) {
    // Trim leading and trailing whitespace from the line
    const trimmedLine = line.trim();
    
    // Skip empty lines or comments starting with #
    if (!trimmedLine || trimmedLine.startsWith("#")) {
      continue;
    }

    // Split the line on the first '=' character
    const [key, value] = trimmedLine.split("=", 2);

    // Check for missing key or no '=' sign
    if (!key || !value) {
      throw new Error(`Invalid config file: ${trimmedLine}`);
    }

    // Trim whitespace around the key and value
    config[key.trim()] = value.trim();
  }
  return config;
}
```