diff --git a/debian/changelog b/debian/changelog
index c16f1844..d7446db4 100644
--- a/debian/changelog
+++ b/debian/changelog
@@ -1,8 +1,16 @@
 llvm-toolchain-9 (1:9.0.1-12) UNRELEASED; urgency=medium
 
+  [ Jessica Clarke ]
   * Cherry-pick upstream patch D74453 to fix atomic compare-and-swap on
     riscv64.
 
+  [ William Grant ]
+  * debian/patches/riscv64-multilib-empty.patch: Adjust riscv64 GCC detector
+    to also check for existence of crtbegin.o in the default multilib dir,
+    like most other ports. Fixes FTBFS on riscv64, since on Ubuntu
+    /usr/lib/gcc/riscv64-linux-gnu/10 exists but is empty (gcc-10-base is
+    installed, but libgcc-10-dev is not).
+
  -- Jessica Clarke <jrtc27@debian.org>  Wed, 01 Apr 2020 16:23:55 +0100
 
 llvm-toolchain-9 (1:9.0.1-11) unstable; urgency=medium
diff --git a/debian/patches/riscv64-multilib-empty.patch b/debian/patches/riscv64-multilib-empty.patch
new file mode 100644
index 00000000..ce605569
--- /dev/null
+++ b/debian/patches/riscv64-multilib-empty.patch
@@ -0,0 +1,51 @@
+Index: llvm-toolchain-9-9.0.1/clang/lib/Driver/ToolChains/Gnu.cpp
+===================================================================
+--- llvm-toolchain-9-9.0.1.orig/clang/lib/Driver/ToolChains/Gnu.cpp
++++ llvm-toolchain-9-9.0.1/clang/lib/Driver/ToolChains/Gnu.cpp
+@@ -1502,11 +1502,12 @@ static bool findMSP430Multilibs(const Dr
+   return false;
+ }
+ 
+-static void findRISCVMultilibs(const Driver &D,
++static bool findRISCVMultilibs(const Driver &D,
+                                const llvm::Triple &TargetTriple, StringRef Path,
+                                const ArgList &Args, DetectedMultilibs &Result) {
+ 
+   FilterNonExistent NonExistent(Path, "/crtbegin.o", D.getVFS());
++  Multilib Default;
+   Multilib Ilp32 = makeMultilib("lib32/ilp32").flag("+m32").flag("+mabi=ilp32");
+   Multilib Ilp32f =
+       makeMultilib("lib32/ilp32f").flag("+m32").flag("+mabi=ilp32f");
+@@ -1517,7 +1518,7 @@ static void findRISCVMultilibs(const Dri
+   Multilib Lp64d = makeMultilib("lib64/lp64d").flag("+m64").flag("+mabi=lp64d");
+   MultilibSet RISCVMultilibs =
+       MultilibSet()
+-          .Either({Ilp32, Ilp32f, Ilp32d, Lp64, Lp64f, Lp64d})
++          .Either({Default, Ilp32, Ilp32f, Ilp32d, Lp64, Lp64f, Lp64d})
+           .FilterOut(NonExistent);
+ 
+   Multilib::flags_list Flags;
+@@ -1533,8 +1534,11 @@ static void findRISCVMultilibs(const Dri
+   addMultilibFlag(ABIName == "lp64f", "mabi=lp64f", Flags);
+   addMultilibFlag(ABIName == "lp64d", "mabi=lp64d", Flags);
+ 
+-  if (RISCVMultilibs.select(Flags, Result.SelectedMultilib))
+-    Result.Multilibs = RISCVMultilibs;
++  if (!RISCVMultilibs.select(Flags, Result.SelectedMultilib))
++    return false;
++
++  Result.Multilibs = RISCVMultilibs;
++  return true;
+ }
+ 
+ static bool findBiarchMultilibs(const Driver &D,
+@@ -2309,7 +2313,8 @@ bool Generic_GCC::GCCInstallationDetecto
+     if (!findMIPSMultilibs(D, TargetTriple, Path, Args, Detected))
+       return false;
+   } else if (TargetTriple.isRISCV()) {
+-    findRISCVMultilibs(D, TargetTriple, Path, Args, Detected);
++    if (!findRISCVMultilibs(D, TargetTriple, Path, Args, Detected))
++      return false;
+   } else if (isMSP430(TargetArch)) {
+     findMSP430Multilibs(D, TargetTriple, Path, Args, Detected);
+   } else if (TargetArch == llvm::Triple::avr) {
diff --git a/debian/patches/series b/debian/patches/series
index b54b9426..f2ff18c5 100644
--- a/debian/patches/series
+++ b/debian/patches/series
@@ -144,3 +144,4 @@ python3-shebang.patch
 print-lldb-path.patch
 no-cgi.patch
 947f9692440836dcb8d88b74b69dd379d85974ce.patch
+riscv64-multilib-empty.patch