diff --git a/.travis.yml b/.travis.yml
index eb7d56054..3375fc846 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -13,20 +13,43 @@ notifications:
 # and an empty 'python' variable in order to prevent that Travis attempts
 # to install Python.
 # TODO: Figure out how specific versions of Python 3.x can be used with OS-X
+
+# When defining the job matrix explicitly, there are Travis environments
+# that produce an additional default job. See these Travis issues:
+#   https://github.com/travis-ci/travis-ci/issues/1228
+#   https://github.com/travis-ci/travis-ci/issues/4681
+#   https://github.com/travis-ci/travis-ci/issues/9843
+# The public Travis does not seem to have this issue anymore,
+# but Travis@IBM does have this issue (as of 9/2018). The workaround for
+# this issue is to define variables globally and to exclude this same
+# variable value in the matrix definition. Experiments have shown that
+# not all variable combinations work. Using a combination of 'language'
+# and 'os' set to the default values works.
+
+# See note about explicit job matrix, above.
+language: ruby
+os: linux
+
 matrix:
+
+  # See note about explicit job matrix, above.
+  exclude:
+    - language: ruby
+    - os: linux
+
   include:
-#    - os: linux
-#      language: python
-#      python: "2.7"
-#      env:
-#        - PACKAGE_LEVEL=minimum
-#      cache: pip
     - os: linux
       language: python
       python: "2.7"
       env:
         - PACKAGE_LEVEL=latest
       cache: pip
+#    - os: linux
+#      language: python
+#      python: "2.7"
+#      env:
+#        - PACKAGE_LEVEL=minimum
+#      cache: pip
     - os: linux
       language: python
       python: "3.4"
@@ -75,13 +98,13 @@ matrix:
 #      env:
 #        - PACKAGE_LEVEL=latest
 #      cache: pip
-    - os: osx
-      language: generic
-      python:
-      env:
-        - PACKAGE_LEVEL=minimum
-        - PYTHON=2
-      cache: pip
+#    - os: osx
+#      language: generic
+#      python:
+#      env:
+#        - PACKAGE_LEVEL=minimum
+#        - PYTHON=2
+#      cache: pip
 #    - os: osx
 #      language: generic
 #      python:
@@ -96,15 +119,16 @@ matrix:
 #        - PACKAGE_LEVEL=minimum
 #        - PYTHON=3
 #      cache: pip
-    - os: osx
-      language: generic
-      python:
-      env:
-        - PACKAGE_LEVEL=latest
-        - PYTHON=3
-      cache: pip
+#    - os: osx
+#      language: generic
+#      python:
+#      env:
+#        - PACKAGE_LEVEL=latest
+#        - PYTHON=3
+#      cache: pip
 
 before_install:
+  - env | sort
   - if [[ "$TRAVIS_BRANCH" == "manual-ci-run" ]]; then
       export _NEED_REBASE=true;
     fi
@@ -115,6 +139,14 @@ before_install:
 
 # commands to install dependencies
 install:
+
+  # The following statement is a safety net in case the matrix exclusion
+  # does not work for some reason.
+  - if [[ "$TRAVIS_LANGUAGE" == "ruby" ]]; then
+      echo "Exiting from unwanted default Ruby job";
+      exit;
+    fi
+
   - if [[ "$TRAVIS_BRANCH" == "manual-ci-run" || "$TRAVIS_PULL_REQUEST_BRANCH" == "manual-ci-run" ]]; then
       export _MANUAL_CI_RUN=true;
     fi