diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index 27ca81553..0736c0212 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -23,7 +23,7 @@ jobs:
     - name: Install and set up Poetry
       run: |
         curl -fsS -o get-poetry.py https://raw.githubusercontent.com/sdispater/poetry/master/get-poetry.py
-        python get-poetry.py --preview -y
+        python get-poetry.py -y
     - name: Build distributions
       run: |
         source $HOME/.poetry/env
diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index e80838a5e..e1ca9947b 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -41,7 +41,7 @@ jobs:
         shell: bash
         run: |
           curl -fsS -o get-poetry.py https://raw.githubusercontent.com/python-poetry/poetry/master/get-poetry.py
-          python get-poetry.py --preview -y
+          python get-poetry.py -y
           echo "$HOME/.poetry/bin" >> $GITHUB_PATH
 
       - name: Configure poetry
diff --git a/poetry/core/packages/constraints/base_constraint.py b/poetry/core/packages/constraints/base_constraint.py
index 8cb95c138..0db9aff42 100644
--- a/poetry/core/packages/constraints/base_constraint.py
+++ b/poetry/core/packages/constraints/base_constraint.py
@@ -6,6 +6,9 @@
 
 
 class BaseConstraint(object):
+    def allows(self, other):  # type: ("ConstraintTypes") -> bool
+        raise NotImplementedError
+
     def allows_all(self, other):  # type: ("ConstraintTypes") -> bool
         raise NotImplementedError()
 
diff --git a/poetry/core/packages/constraints/empty_constraint.py b/poetry/core/packages/constraints/empty_constraint.py
index eb29e4ad3..4db043def 100644
--- a/poetry/core/packages/constraints/empty_constraint.py
+++ b/poetry/core/packages/constraints/empty_constraint.py
@@ -17,6 +17,9 @@ def matches(self, _):  # type: ("ConstraintTypes") -> bool
     def is_empty(self):  # type: () -> bool
         return True
 
+    def allows(self, other):  # type: ("ConstraintTypes") -> bool
+        return False
+
     def allows_all(self, other):  # type: ("ConstraintTypes") -> bool
         return True
 
diff --git a/poetry/core/version/markers.py b/poetry/core/version/markers.py
index 894e1dc17..ac6c2b60f 100644
--- a/poetry/core/version/markers.py
+++ b/poetry/core/version/markers.py
@@ -225,7 +225,17 @@ def __init__(
             else:
                 self._constraint = self._parser(self._constraint_string)
         else:
-            self._constraint = self._parser(self._constraint_string)
+            # if we have a in/not in operator we split the constraint
+            # into a union/multi-constraint of single constraint
+            constraint_string = self._constraint_string
+            if self._operator in {"in", "not in"}:
+                op, glue = ("==", " || ") if self._operator == "in" else ("!=", ", ")
+                values = re.split("[ ,]+", self._value)
+                constraint_string = glue.join(
+                    ("{} {}".format(op, value) for value in values)
+                )
+
+            self._constraint = self._parser(constraint_string)
 
     @property
     def name(self):  # type: () -> str
diff --git a/tests/version/test_markers.py b/tests/version/test_markers.py
index f31bd4442..389d25c13 100644
--- a/tests/version/test_markers.py
+++ b/tests/version/test_markers.py
@@ -36,6 +36,34 @@ def test_single_marker():
     assert m.constraint_string == "not in 2.7, 3.0, 3.1"
     assert str(m.constraint) == "<2.7.0 || >=2.8.0,<3.0.0 || >=3.2.0"
 
+    m = parse_marker(
+        "platform_machine in 'x86_64 X86_64 aarch64 AARCH64 ppc64le PPC64LE amd64 AMD64 win32 WIN32'"
+    )
+
+    assert isinstance(m, SingleMarker)
+    assert m.name == "platform_machine"
+    assert (
+        m.constraint_string
+        == "in x86_64 X86_64 aarch64 AARCH64 ppc64le PPC64LE amd64 AMD64 win32 WIN32"
+    )
+    assert str(m.constraint) == (
+        "x86_64 || X86_64 || aarch64 || AARCH64 || ppc64le || PPC64LE || amd64 || AMD64 || win32 || WIN32"
+    )
+
+    m = parse_marker(
+        "platform_machine not in 'x86_64 X86_64 aarch64 AARCH64 ppc64le PPC64LE amd64 AMD64 win32 WIN32'"
+    )
+
+    assert isinstance(m, SingleMarker)
+    assert m.name == "platform_machine"
+    assert (
+        m.constraint_string
+        == "not in x86_64 X86_64 aarch64 AARCH64 ppc64le PPC64LE amd64 AMD64 win32 WIN32"
+    )
+    assert str(m.constraint) == (
+        "!=x86_64, !=X86_64, !=aarch64, !=AARCH64, !=ppc64le, !=PPC64LE, !=amd64, !=AMD64, !=win32, !=WIN32"
+    )
+
 
 def test_single_marker_intersect():
     m = parse_marker('sys_platform == "darwin"')
@@ -476,6 +504,26 @@ def test_multi_marker_removes_duplicates():
             {"python_version": "2.7"},
             False,
         ),
+        (
+            "platform_machine in 'x86_64 X86_64 aarch64 AARCH64 ppc64le PPC64LE amd64 AMD64 win32 WIN32'",
+            {"platform_machine": "foo"},
+            False,
+        ),
+        (
+            "platform_machine in 'x86_64 X86_64 aarch64 AARCH64 ppc64le PPC64LE amd64 AMD64 win32 WIN32'",
+            {"platform_machine": "x86_64"},
+            True,
+        ),
+        (
+            "platform_machine not in 'x86_64 X86_64 aarch64 AARCH64 ppc64le PPC64LE amd64 AMD64 win32 WIN32'",
+            {"platform_machine": "foo"},
+            True,
+        ),
+        (
+            "platform_machine not in 'x86_64 X86_64 aarch64 AARCH64 ppc64le PPC64LE amd64 AMD64 win32 WIN32'",
+            {"platform_machine": "x86_64"},
+            False,
+        ),
     ],
 )
 def test_validate(marker_string, environment, expected):