diff --git a/src/pip/_internal/resolution/resolvelib/base.py b/src/pip/_internal/resolution/resolvelib/base.py
index 57013b7b2..03d297ba2 100644
--- a/src/pip/_internal/resolution/resolvelib/base.py
+++ b/src/pip/_internal/resolution/resolvelib/base.py
@@ -55,7 +55,7 @@ class Candidate(object):
         raise NotImplementedError("Override in subclass")
 
     def iter_dependencies(self):
-        # type: () -> Iterable[Requirement]
+        # type: () -> Iterable[Optional[Requirement]]
         raise NotImplementedError("Override in subclass")
 
     def get_install_requirement(self):
diff --git a/src/pip/_internal/resolution/resolvelib/candidates.py b/src/pip/_internal/resolution/resolvelib/candidates.py
index 695b42a96..58684e832 100644
--- a/src/pip/_internal/resolution/resolvelib/candidates.py
+++ b/src/pip/_internal/resolution/resolvelib/candidates.py
@@ -217,7 +217,7 @@ class _InstallRequirementBackedCandidate(Candidate):
         return spec
 
     def iter_dependencies(self):
-        # type: () -> Iterable[Requirement]
+        # type: () -> Iterable[Optional[Requirement]]
         for r in self.dist.requires():
             yield self._factory.make_requirement_from_spec(str(r), self._ireq)
         python_dep = self._factory.make_requires_python_requirement(
@@ -343,7 +343,7 @@ class AlreadyInstalledCandidate(Candidate):
         return self.dist.parsed_version
 
     def iter_dependencies(self):
-        # type: () -> Iterable[Requirement]
+        # type: () -> Iterable[Optional[Requirement]]
         for r in self.dist.requires():
             yield self._factory.make_requirement_from_spec(str(r), self._ireq)
 
@@ -425,7 +425,7 @@ class ExtrasCandidate(Candidate):
         return self.base.is_installed
 
     def iter_dependencies(self):
-        # type: () -> Iterable[Requirement]
+        # type: () -> Iterable[Optional[Requirement]]
         factory = self.base._factory
 
         # The user may have specified extras that the candidate doesn't
@@ -486,7 +486,7 @@ class RequiresPythonCandidate(Candidate):
         return self._version
 
     def iter_dependencies(self):
-        # type: () -> Iterable[Requirement]
+        # type: () -> Iterable[Optional[Requirement]]
         return ()
 
     def get_install_requirement(self):
diff --git a/src/pip/_internal/resolution/resolvelib/provider.py b/src/pip/_internal/resolution/resolvelib/provider.py
index 98b9f9420..72f162059 100644
--- a/src/pip/_internal/resolution/resolvelib/provider.py
+++ b/src/pip/_internal/resolution/resolvelib/provider.py
@@ -147,4 +147,4 @@ class PipProvider(AbstractProvider):
         # type: (Candidate) -> Sequence[Requirement]
         if self._ignore_dependencies:
             return []
-        return list(candidate.iter_dependencies())
+        return [r for r in candidate.iter_dependencies() if r is not None]