diff --git a/shell/completion.bash b/shell/completion.bash
index 3589580a..2132ffd1 100644
--- a/shell/completion.bash
+++ b/shell/completion.bash
@@ -529,7 +529,10 @@ if ! declare -F __fzf_list_hosts > /dev/null; then
       ) \
       <(
         __fzf_exec_awk '
-          /^[[:blank:]]*(#|$)|0\.0\.0\.0/ { next }
+          # Note: mawk <= 1.3.3-20090705 does not support the POSIX brackets of
+          # the form [[:blank:]], and Ubuntu 18.04 LTS still uses this
+          # 16-year-old mawk unfortunately.  We need to use [ \t] instead.
+          /^[ \t]*(#|$)|0\.0\.0\.0/ { next }
           {
             sub(/#.*/, "")
             for (i = 2; i <= NF; i++)
diff --git a/shell/completion.zsh b/shell/completion.zsh
index 7f4a8112..d5406880 100644
--- a/shell/completion.zsh
+++ b/shell/completion.zsh
@@ -307,7 +307,10 @@ if ! declare -f __fzf_list_hosts > /dev/null; then
       ) \
       <(
         __fzf_exec_awk '
-          /^[[:blank:]]*(#|$)|0\.0\.0\.0/ { next }
+          # Note: mawk <= 1.3.3-20090705 does not support the POSIX brackets of
+          # the form [[:blank:]], and Ubuntu 18.04 LTS still uses this
+          # 16-year-old mawk unfortunately.  We need to use [ \t] instead.
+          /^[ \t]*(#|$)|0\.0\.0\.0/ { next }
           {
             sub(/#.*/, "")
             for (i = 2; i <= NF; i++)