Index: llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
===================================================================
--- llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -467,7 +467,9 @@
     return isUnsignedDIType(DD, DD->resolve(Deriv));
   }
 
-  auto *BTy = cast<DIBasicType>(Ty);
+  auto *BTy = dyn_cast<DIBasicType>(Ty);
+  if (!BTy)
+    return false;
   unsigned Encoding = BTy->getEncoding();
   assert((Encoding == dwarf::DW_ATE_unsigned ||
           Encoding == dwarf::DW_ATE_unsigned_char ||
Index: llvm/test/Assembler/DIAsmunsigned.ll
===================================================================
--- /dev/null
+++ llvm/test/Assembler/DIAsmunsigned.ll
@@ -0,0 +1,42 @@
+; RUN: llc < %s | FileCheck %s
+; CHECK:        .section        __DWARF,__debug_str,regular,debug
+; CHECK-NEXT: Linfo_string:
+; CHECK_NEXT:        .asciz  "zig 0.3.0"             ## string offset=0
+
+
+%timespec.0.1.2.3.0.1.2 = type { i64, i64 }
+define internal i64 @init_vdso_clock_gettime(i32, %timespec.0.1.2.3.0.1.2* nonnull) unnamed_addr !dbg !142 {
+  call void @llvm.dbg.value(metadata i64 (i32, %timespec.0.1.2.3.0.1.2*)* null, metadata !162, metadata !DIExpression()), !dbg !167
+  ret i64 -38, !dbg !168
+}
+declare void @llvm.dbg.value(metadata, metadata, metadata) #0
+!llvm.module.flags = !{!0}
+!llvm.dbg.cu = !{!1}
+!0 = !{i32 2, !"Debug Info Version", i32 3}
+!1 = distinct !DICompileUnit(language: DW_LANG_C99, file: !2, producer: "zig 0.3.0", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !3, globals: !76)
+!2 = !DIFile(filename: "test", directory: ".")
+!3 = !{!4}
+!4 = !DICompositeType(tag: DW_TAG_enumeration_type, name: "Arch", scope: !5, file: !5, line: 44, baseType: !6, size: 8, align: 8, elements: !7)
+!5 = !DIFile(filename: "builtin.zig", directory: "/home/andy/.local/share/zig/stage1/builtin/ugMGxVES9OkDAffv3xhJS3KQVy0Wm1xPM3Bc6x4MBuup5aetdi5pVTrGRG2aDAn0")
+!6 = !DIBasicType(name: "u7", size: 8, encoding: DW_ATE_unsigned)
+!7 = !{!8}
+!8 = !DIEnumerator(name: "armv8_5a", value: 0)
+!76 = !{!77}
+!77 = !DIGlobalVariableExpression(var: !78, expr: !DIExpression())
+!78 = distinct !DIGlobalVariable(name: "arch", linkageName: "arch", scope: !5, file: !5, line: 437, type: !4, isLocal: true, isDefinition: true)
+!81 = !DIFile(filename: "index.zig", directory: "/store/dev/zig/build-llvm8-debug/lib/zig/std/os/linux")
+!142 = distinct !DISubprogram(name: "init_vdso_clock_gettime", scope: !81, file: !81, line: 968, type: !143, scopeLine: 968, flags: DIFlagStaticMember, spFlags: DISPFlagLocalToUnit | DISPFlagDefinition | DISPFlagOptimized, unit: !1, retainedNodes: !153)
+!143 = !DISubroutineType(types: !144)
+!144 = !{!145}
+!145 = !DIBasicType(name: "usize", size: 64, encoding: DW_ATE_unsigned)
+!146 = !DIBasicType(name: "i32", size: 32, encoding: DW_ATE_signed)
+!153 = !{!154}
+!154 = !DILocalVariable(name: "clk", arg: 1, scope: !142, file: !81, line: 968, type: !146)
+!162 = !DILocalVariable(name: "f", scope: !163, file: !81, line: 970, type: !143)
+!163 = distinct !DILexicalBlock(scope: !164, file: !81, line: 969, column: 5)
+!164 = distinct !DILexicalBlock(scope: !165, file: !81, line: 968, column: 66)
+!165 = distinct !DILexicalBlock(scope: !166, file: !81, line: 968, column: 45)
+!166 = distinct !DILexicalBlock(scope: !142, file: !81, line: 968, column: 35)
+!167 = !DILocation(line: 970, column: 5, scope: !163)
+!168 = !DILocation(line: 972, column: 28, scope: !169)
+!169 = distinct !DILexicalBlock(scope: !163, file: !81, line: 970, column: 5)