diff --git a/Lib/fontTools/varLib/__init__.py b/Lib/fontTools/varLib/__init__.py
index 3efb2548c..313b1a398 100644
--- a/Lib/fontTools/varLib/__init__.py
+++ b/Lib/fontTools/varLib/__init__.py
@@ -512,13 +512,28 @@ def _add_MVAR(font, masterModel, master_ttfs, axisTags):
lastTableTag = None
fontTable = None
tables = None
+ # HACK: we need to special-case post.underlineThickness and .underlinePosition
+ # and unilaterally/arbitrarily define a sentinel value to distinguish the case
+ # when a post table is present in a given master simply because that's where
+ # the glyph names in TrueType must be stored, but the underline values are not
+ # meant to be used for building MVAR's deltas. The value of -0x8000 (-36768)
+ # the minimum FWord (int16) value, was chosen for its unlikelyhood to appear
+ # in real-world underline position/thickness values.
+ specialTags = {"unds": -0x8000, "undo": -0x8000}
for tag, (tableTag, itemName) in sorted(MVAR_ENTRIES.items(), key=lambda kv: kv[1]):
if tableTag != lastTableTag:
tables = fontTable = None
if tableTag in font:
fontTable = font[tableTag]
- tables = [master[tableTag] if tableTag in master else None
- for master in master_ttfs]
+ tables = []
+ for master in master_ttfs:
+ if tableTag not in master or (
+ tag in specialTags
+ and getattr(master[tableTag], itemName) == specialTags[tag]
+ ):
+ tables.append(None)
+ else:
+ tables.append(master[tableTag])
lastTableTag = tableTag
if tables is None:
continue
diff --git a/Tests/varLib/data/SparseMasters.designspace b/Tests/varLib/data/SparseMasters.designspace
new file mode 100644
index 000000000..327fb43b9
--- /dev/null
+++ b/Tests/varLib/data/SparseMasters.designspace
@@ -0,0 +1,23 @@
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diff --git a/Tests/varLib/data/master_ttx_interpolatable_ttf/SparseMasters-Bold.ttx b/Tests/varLib/data/master_ttx_interpolatable_ttf/SparseMasters-Bold.ttx
new file mode 100644
index 000000000..55d686e3d
--- /dev/null
+++ b/Tests/varLib/data/master_ttx_interpolatable_ttf/SparseMasters-Bold.ttx
@@ -0,0 +1,419 @@
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+
+
+ Layer Font Bold
+
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+ Version 0.000
+
+
+ LayerFont-Bold
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diff --git a/Tests/varLib/data/master_ttx_interpolatable_ttf/SparseMasters-Medium.ttx b/Tests/varLib/data/master_ttx_interpolatable_ttf/SparseMasters-Medium.ttx
new file mode 100644
index 000000000..eb3c74547
--- /dev/null
+++ b/Tests/varLib/data/master_ttx_interpolatable_ttf/SparseMasters-Medium.ttx
@@ -0,0 +1,125 @@
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diff --git a/Tests/varLib/data/master_ttx_interpolatable_ttf/SparseMasters-Regular.ttx b/Tests/varLib/data/master_ttx_interpolatable_ttf/SparseMasters-Regular.ttx
new file mode 100644
index 000000000..e013e0b7e
--- /dev/null
+++ b/Tests/varLib/data/master_ttx_interpolatable_ttf/SparseMasters-Regular.ttx
@@ -0,0 +1,419 @@
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+
+ 0.000;NONE;LayerFont-Regular
+
+
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+
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+ Version 0.000
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diff --git a/Tests/varLib/data/test_results/SparseMasters.ttx b/Tests/varLib/data/test_results/SparseMasters.ttx
new file mode 100644
index 000000000..99a80bfb5
--- /dev/null
+++ b/Tests/varLib/data/test_results/SparseMasters.ttx
@@ -0,0 +1,660 @@
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