diff --git a/core/fpdfdoc/cpdf_occontext.cpp b/core/fpdfdoc/cpdf_occontext.cpp
index 66950c9..7e746e0 100644
--- a/core/fpdfdoc/cpdf_occontext.cpp
+++ b/core/fpdfdoc/cpdf_occontext.cpp
@@ -254,18 +254,25 @@
     return true;
 
   bool bState = (csP == "AllOn" || csP == "AllOff");
+  // At least one entry of OCGs needs to be a valid dictionary for it to be
+  // considered present. See "OCGs" in table 4.49 in the PDF 1.7 spec.
+  bool bValidEntrySeen = false;
   for (size_t i = 0; i < pArray->GetCount(); i++) {
     bool bItem = true;
     CPDF_Dictionary* pItemDict = pArray->GetDictAt(i);
-    if (pItemDict)
-      bItem = GetOCGVisible(pItemDict);
+    if (!pItemDict)
+      continue;
+
+    bValidEntrySeen = true;
+    bItem = GetOCGVisible(pItemDict);
 
     if ((csP == "AnyOn" && bItem) || (csP == "AnyOff" && !bItem))
       return true;
     if ((csP == "AllOn" && !bItem) || (csP == "AllOff" && bItem))
       return false;
   }
-  return bState;
+
+  return !bValidEntrySeen || bState;
 }
 
 bool CPDF_OCContext::CheckOCGVisible(const CPDF_Dictionary* pOCGDict) {
diff --git a/testing/resources/pixel/bug_491_invisible.in b/testing/resources/pixel/bug_491_invisible.in
new file mode 100644
index 0000000..75b7e58
--- /dev/null
+++ b/testing/resources/pixel/bug_491_invisible.in
@@ -0,0 +1,75 @@
+{{header}}
+{{object 1 0}} <<
+  /Type /Catalog
+  /Pages 2 0 R
+  /OCProperties <<
+    /OCGs [
+      8 0 R
+    ]
+    /D <<
+      /BaseState /OFF
+    >>
+  >>
+>>
+{{object 2 0}} <<
+  /Type /Pages
+  /MediaBox [ 0 0 200 200 ]
+  /Count 1
+  /Kids [ 3 0 R ]
+>>
+endobj
+{{object 3 0}} <<
+  /Type /Page
+  /Parent 2 0 R
+  /Resources <<
+    /Font <<
+      /F1 4 0 R
+      /F2 5 0 R
+    >>
+    /Properties <<
+      /OC0 7 0 R
+    >>
+  >>
+  /Contents 6 0 R
+>>
+endobj
+{{object 4 0}} <<
+  /Type /Font
+  /Subtype /Type1
+  /BaseFont /Times-Roman
+>>
+endobj
+{{object 5 0}} <<
+  /Type /Font
+  /Subtype /Type1
+  /BaseFont /Helvetica
+>>
+endobj
+{{object 6 0}} <<
+>>
+stream
+16 96 32 32 re B
+/OC /OC0 BDC
+64 96 32 32 re B
+EMC
+endstream
+endobj
+{{object 7 0}} <<
+  /OCGs [
+    8 0 R
+  ]
+  /Type /OCMD
+>>
+endobj
+{{object 8 0}} <<
+  /Type /OCG
+  /Name (Show Text)
+>>
+endobj
+{{xref}}
+trailer <<
+  /Size 9
+  /Root 1 0 R
+>>
+{{startxref}}
+%%EOF
diff --git a/testing/resources/pixel/bug_491_invisible_expected.pdf.0.png b/testing/resources/pixel/bug_491_invisible_expected.pdf.0.png
new file mode 100644
index 0000000..067ca22f
--- /dev/null
+++ b/testing/resources/pixel/bug_491_invisible_expected.pdf.0.png
Binary files differ
diff --git a/testing/resources/pixel/bug_491_unspecified.in b/testing/resources/pixel/bug_491_unspecified.in
new file mode 100644
index 0000000..b1e7312
--- /dev/null
+++ b/testing/resources/pixel/bug_491_unspecified.in
@@ -0,0 +1,74 @@
+{{header}}
+{{object 1 0}} <<
+  /Type /Catalog
+  /Pages 2 0 R
+  /OCProperties <<
+    /OCGs [
+      8 0 R
+    ]
+    /D <<
+      /BaseState /OFF
+    >>
+  >>
+>>
+{{object 2 0}} <<
+  /Type /Pages
+  /MediaBox [ 0 0 200 200 ]
+  /Count 1
+  /Kids [ 3 0 R ]
+>>
+endobj
+{{object 3 0}} <<
+  /Type /Page
+  /Parent 2 0 R
+  /Resources <<
+    /Font <<
+      /F1 4 0 R
+      /F2 5 0 R
+    >>
+    /Properties <<
+      /OC0 7 0 R
+    >>
+  >>
+  /Contents 6 0 R
+>>
+endobj
+{{object 4 0}} <<
+  /Type /Font
+  /Subtype /Type1
+  /BaseFont /Times-Roman
+>>
+endobj
+{{object 5 0}} <<
+  /Type /Font
+  /Subtype /Type1
+  /BaseFont /Helvetica
+>>
+endobj
+{{object 6 0}} <<
+>>
+stream
+16 96 32 32 re B
+/OC /OC0 BDC
+64 96 32 32 re B
+EMC
+endstream
+endobj
+{{object 7 0}} <<
+  /OCGs [
+  ]
+  /Type /OCMD
+>>
+endobj
+{{object 8 0}} <<
+  /Type /OCG
+  /Name (Show Text)
+>>
+endobj
+{{xref}}
+trailer <<
+  /Size 9
+  /Root 1 0 R
+>>
+{{startxref}}
+%%EOF
diff --git a/testing/resources/pixel/bug_491_unspecified_expected.pdf.0.png b/testing/resources/pixel/bug_491_unspecified_expected.pdf.0.png
new file mode 100644
index 0000000..fa1b37c
--- /dev/null
+++ b/testing/resources/pixel/bug_491_unspecified_expected.pdf.0.png
Binary files differ
diff --git a/testing/resources/pixel/bug_491_visible.in b/testing/resources/pixel/bug_491_visible.in
new file mode 100644
index 0000000..8e21b6c
--- /dev/null
+++ b/testing/resources/pixel/bug_491_visible.in
@@ -0,0 +1,75 @@
+{{header}}
+{{object 1 0}} <<
+  /Type /Catalog
+  /Pages 2 0 R
+  /OCProperties <<
+    /OCGs [
+      8 0 R
+    ]
+    /D <<
+      /BaseState /ON
+    >>
+  >>
+>>
+{{object 2 0}} <<
+  /Type /Pages
+  /MediaBox [ 0 0 200 200 ]
+  /Count 1
+  /Kids [ 3 0 R ]
+>>
+endobj
+{{object 3 0}} <<
+  /Type /Page
+  /Parent 2 0 R
+  /Resources <<
+    /Font <<
+      /F1 4 0 R
+      /F2 5 0 R
+    >>
+    /Properties <<
+      /OC0 7 0 R
+    >>
+  >>
+  /Contents 6 0 R
+>>
+endobj
+{{object 4 0}} <<
+  /Type /Font
+  /Subtype /Type1
+  /BaseFont /Times-Roman
+>>
+endobj
+{{object 5 0}} <<
+  /Type /Font
+  /Subtype /Type1
+  /BaseFont /Helvetica
+>>
+endobj
+{{object 6 0}} <<
+>>
+stream
+16 96 32 32 re B
+/OC /OC0 BDC
+64 96 32 32 re B
+EMC
+endstream
+endobj
+{{object 7 0}} <<
+  /OCGs [
+    8 0 R
+  ]
+  /Type /OCMD
+>>
+endobj
+{{object 8 0}} <<
+  /Type /OCG
+  /Name (Show Text)
+>>
+endobj
+{{xref}}
+trailer <<
+  /Size 9
+  /Root 1 0 R
+>>
+{{startxref}}
+%%EOF
diff --git a/testing/resources/pixel/bug_491_visible_expected.pdf.0.png b/testing/resources/pixel/bug_491_visible_expected.pdf.0.png
new file mode 100644
index 0000000..fa1b37c
--- /dev/null
+++ b/testing/resources/pixel/bug_491_visible_expected.pdf.0.png
Binary files differ