diff --git a/subiquity/common/filesystem/tests/test_gaps.py b/subiquity/common/filesystem/tests/test_gaps.py
index 4d8e2749..18908627 100644
--- a/subiquity/common/filesystem/tests/test_gaps.py
+++ b/subiquity/common/filesystem/tests/test_gaps.py
@@ -270,7 +270,7 @@ class TestDiskGaps(unittest.TestCase):
         info = PartitionAlignmentData(
             part_align=5, min_gap_size=1, min_start_offset=0, min_end_offset=0,
             ebr_space=2, primary_part_limit=10)
-        m, d = make_model_and_disk(size=100)
+        m, d = make_model_and_disk(size=100, ptable='dos')
         p1 = make_partition(m, d, offset=0, size=50, flag='extended')
         p2 = make_partition(m, d, offset=5, size=45, flag='logical')
         self.assertEqual(
@@ -364,7 +364,7 @@ class TestMovableTrailingPartitionsAndGapSize(unittest.TestCase):
         # 0----10---20---30---40---50---60---70---80---90---100
         # #####[ p1 (extended)    ]                    #####
         # ######[ p5 (logical)    ]                    #####
-        m, d = make_model_and_disk(size=100)
+        m, d = make_model_and_disk(size=100, ptable='dos')
         make_partition(m, d, offset=10, size=40, flag='extended')
         p5 = make_partition(m, d, offset=12, size=38, flag='logical')
         self.assertEqual(
@@ -378,7 +378,7 @@ class TestMovableTrailingPartitionsAndGapSize(unittest.TestCase):
         # 0----10---20---30---40---50---60---70---80---90---100
         # #####[ p1 (extended)    ][ p2               ]#####
         # ######[ p5 (logical)    ]                    #####
-        m, d = make_model_and_disk(size=100)
+        m, d = make_model_and_disk(size=100, ptable='dos')
         make_partition(m, d, offset=10, size=40, flag='extended')
         make_partition(m, d, offset=50, size=40)
         p5 = make_partition(m, d, offset=12, size=38, flag='logical')
@@ -393,7 +393,7 @@ class TestMovableTrailingPartitionsAndGapSize(unittest.TestCase):
         # 0----10---20---30---40---50---60---70---80---90---100
         # #####[ p1 (extended)    ]                    #####
         # ######[ p5 (logical)]                        #####
-        m, d = make_model_and_disk(size=100)
+        m, d = make_model_and_disk(size=100, ptable='dos')
         make_partition(m, d, offset=10, size=40, flag='extended')
         p5 = make_partition(m, d, offset=12, size=30, flag='logical')
         self.assertEqual(
@@ -407,7 +407,7 @@ class TestMovableTrailingPartitionsAndGapSize(unittest.TestCase):
         # 0----10---20---30---40---50---60---70---80---90---100
         # #####[ p1 (extended)                        ]#####
         # ######[ p5 (logical)] [ p6 (logical)]        #####
-        m, d = make_model_and_disk(size=100)
+        m, d = make_model_and_disk(size=100, ptable='dos')
         make_partition(m, d, offset=10, size=80, flag='extended')
         p5 = make_partition(m, d, offset=12, size=30, flag='logical')
         p6 = make_partition(m, d, offset=44, size=30, flag='logical')
@@ -422,7 +422,7 @@ class TestMovableTrailingPartitionsAndGapSize(unittest.TestCase):
         # 0----10---20---30---40---50---60---70---80---90---100
         # #####[ p1 (extended)                        ]#####
         # ######[ p5 (logical)] [ p6 (logical)       ] #####
-        m, d = make_model_and_disk(size=100)
+        m, d = make_model_and_disk(size=100, ptable='dos')
         make_partition(m, d, offset=10, size=80, flag='extended')
         p5 = make_partition(m, d, offset=12, size=30, flag='logical')
         p6 = make_partition(m, d, offset=44, size=44, flag='logical')
diff --git a/subiquity/models/filesystem.py b/subiquity/models/filesystem.py
index f949d08a..1a159e28 100644
--- a/subiquity/models/filesystem.py
+++ b/subiquity/models/filesystem.py
@@ -688,11 +688,24 @@ class Partition(_Formattable):
     def __post_init__(self):
         if self.number is not None:
             return
-        used_nums = {part.number for part in self.device._partitions
-                     if part.number is not None}
-        possible_nums = {i for i in range(1, len(self.device._partitions) + 1)}
-        unused_nums = sorted(list(possible_nums - used_nums))
-        self.number = unused_nums.pop(0)
+
+        def is_logical(part):
+            return part.flag == 'logical'
+
+        primary_limit = self.device.alignment_data().primary_part_limit
+        used_nums = {p.number for p in self.device._partitions
+                     if p.number is not None
+                     if is_logical(p) == is_logical(self)}
+        if is_logical(self):
+            possibles = range(primary_limit + 1, 129)
+        else:
+            possibles = range(1, primary_limit + 1)
+
+        for num in possibles:
+            if num not in used_nums:
+                self.number = num
+                return
+        raise Exception('Failed to allocate partition number')
 
     def available(self):
         if self.flag in ['bios_grub', 'prep'] or self.grub_device: