diff --git a/prism/util/pm_string.c b/prism/util/pm_string.c
index dfc121b6a2..e9e597113f 100644
--- a/prism/util/pm_string.c
+++ b/prism/util/pm_string.c
@@ -116,6 +116,13 @@ pm_string_mapped_init(pm_string_t *string, const char *filepath) {
         return false;
     }
 
+    // Ensure it is a file and not a directory
+    if (S_ISDIR(sb.st_mode)) {
+        errno = EISDIR;
+        close(fd);
+        return false;
+    }
+
     // mmap the file descriptor to virtually get the contents
     size_t size = (size_t) sb.st_size;
     uint8_t *source = NULL;
diff --git a/prism/util/pm_string.h b/prism/util/pm_string.h
index d23792c0ba..e4a20558d3 100644
--- a/prism/util/pm_string.h
+++ b/prism/util/pm_string.h
@@ -9,6 +9,7 @@
 #include "prism/defines.h"
 
 #include <assert.h>
+#include <errno.h>
 #include <stdbool.h>
 #include <stddef.h>
 #include <stdlib.h>