diff --git a/src/include/units/bits/fixed_string.h b/src/include/units/bits/fixed_string.h
index 88357186..719227a4 100644
--- a/src/include/units/bits/fixed_string.h
+++ b/src/include/units/bits/fixed_string.h
@@ -20,6 +20,8 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#pragma once
+
 #include <cstdlib>
 #include <ostream>