diff --git a/src/pip/_internal/req/req_install.py b/src/pip/_internal/req/req_install.py
index cd64ef565..77526283d 100644
--- a/src/pip/_internal/req/req_install.py
+++ b/src/pip/_internal/req/req_install.py
@@ -580,8 +580,18 @@ class InstallRequirement(object):
         if "build-system" not in pp_toml:
             return ["setuptools", "wheel"]
 
+        # Specifying the build-system table but not the requires key is invalid
+        build_system = pp_toml["build-system"]
+        if "requires" not in build_system:
+            raise InstallationError(
+                error_template.format(package=self, reason=(
+                    "it has a 'build-system' table but not the "
+                    "'build-system.requires' which is mandatory in the table"
+                ))
+            )
+
         # Error out if it's not a list of strings
-        requires = build_system.get("requires", [])
+        requires = build_system["requires"]
         if not _is_list_of_str(requires):
             raise InstallationError(error_template.format(
                 self, "'build-system.requires' is not a list of strings."